Add an use new strtou?ll?() wrappers in webget.c

This is kind of a prototype for xpdev... but it's a bit klunky.

The issue these fix is that some C libraries (Windows and GLibC)
will return 0 and leave errno unset when no digits are converted,
while other libraries (FreeBSD and musl) will set errno if no
digits are converted.  This means that even setting errno to zero
before calling, then checking it if the return value is zero is
not enough to catch errors.

This actually led to Windows and (most) Linux systems not being able
to download the web lists because of a bug where it was converting
starting in the middle of the string rather than where the numbers
are.  It worked fine on Alpine Linux and FreeBSD though.

src/syncterm/webget.c

@@ -49,6 +49,57 @@ struct http_session {
 	bool got_size;
 };
 
+static bool
+paranoid_strtol(const char *nptr, char ** endptr, int base, long *result)
+{
+	char *extra_endptr;
+	if (endptr == NULL)
+		endptr = &extra_endptr;
+	errno = 0;
+	*result = strtol(nptr, endptr, base);
+	if (*result == 0) {
+		if (errno)
+			return false;
+		if (*endptr == nptr)
+			return false;
+	}
+	return true;
+}
+
+static bool
+paranoid_strtoll(const char *nptr, char ** endptr, int base, long long *result)
+{
+	char *extra_endptr;
+	if (endptr == NULL)
+		endptr = &extra_endptr;
+	errno = 0;
+	*result = strtoll(nptr, endptr, base);
+	if (*result == 0) {
+		if (errno)
+			return false;
+		if (*endptr == nptr)
+			return false;
+	}
+	return true;
+}
+
+static bool
+paranoid_strtoul(const char *nptr, char ** endptr, int base, unsigned long *result)
+{
+	char *extra_endptr;
+	if (endptr == NULL)
+		endptr = &extra_endptr;
+	errno = 0;
+	*result = strtoul(nptr, endptr, base);
+	if (*result == 0) {
+		if (errno)
+			return false;
+		if (*endptr == nptr)
+			return false;
+	}
+	return true;
+}
+
 static void
 set_state_locked(struct webget_request *req, const char *newstate)
 {
@@ -384,7 +435,8 @@ parse_http_date(const char *dstr)
 		t.tm_wday = 0;
 	else
 		return 0;
-	l = strtol(&p[5], &end, 10);
+	if (!paranoid_strtol(&p[5], &end, 10, &l))
+		return 0;
 	if (l < 1 || l > 31)
 		return 0;
 	if (*end != ' ')
@@ -414,33 +466,28 @@ parse_http_date(const char *dstr)
 		t.tm_mon = 10;
 	else if (memcmp(&p[8], "Dec ", 4) == 0)
 		t.tm_mon = 11;
-	l = strtol(&p[12], &end, 10);
+	if (!paranoid_strtol(&p[12], &end, 10, &l))
+		return 0;
 	if (*end != ' ')
 		return 0;
 	if (l < 1900 || l > 5000)
 		return 0;
 	t.tm_year = l - 1900;
-	errno = 0;
-	l = strtol(&p[17], &end, 10);
-	if (l == 0 && errno)
+	if (!paranoid_strtol(&p[17], &end, 10, &l))
 		return 0;
 	if (l < 0 || l > 23)
 		return 0;
 	if (*end != ':')
 		return 0;
 	t.tm_hour = l;
-	errno = 0;
-	l = strtol(&p[20], &end, 10);
-	if (l == 0 && errno)
+	if (!paranoid_strtol(&p[20], &end, 10, &l))
 		return 0;
 	if (l < 0 || l > 59)
 		return 0;
 	if (*end != ':')
 		return 0;
 	t.tm_min = l;
-	errno = 0;
-	l = strtol(&p[23], &end, 10);
-	if (l == 0 && errno)
+	if (!paranoid_strtol(&p[23], &end, 10, &l))
 		return 0;
 	if (l < 0 || l > 60)
 		return 0;
@@ -535,10 +582,9 @@ parse_cache_control(struct http_session *sess, const char *val)
 		// The sep check is for Coverity...
 		if (sz == 7 && strnicmp(val, "max-age=", 8) == 0 && sep) {
 			errno = 0;
-			long long ll = strtoll(&sep[0], NULL, 10);
-			if (ll != 0 || errno == 0) {
+			long long ll;
+			if (paranoid_strtoll(&sep[0], NULL, 10, &ll))
 				sess->cache.max_age = ll;
-			}
 		}
 		if (sz == 8 && sep == NULL && strnicmp(val, "no-cache", 8) == 0) {
 			sess->cache.no_cache = true;
@@ -628,9 +674,8 @@ parse_headers(struct http_session *sess)
 		}
 		else if(strnicmp(line, "age:", 4) == 0) {
 			if (sess->cache.age == 0) {
-				errno = 0;
-				long long ll = strtoll(&line[4], NULL, 10);
-				if (ll > 0 || errno == 0)
+				long long ll;
+				if (paranoid_strtoll(&line[4], NULL, 10, &ll))
 					sess->cache.age = ll;
 			}
 		}
@@ -639,9 +684,8 @@ parse_headers(struct http_session *sess)
 				goto error_return;
 		}
 		else if(strnicmp(line, "content-length:", 15) == 0) {
-			errno = 0;
-			long long ll = strtoll(&line[15], NULL, 10);
-			if (ll > 0 || errno == 0) {
+			long long ll;
+			if (paranoid_strtoll(&line[15], NULL, 10, &ll)) {
 				if (ll > MAX_LIST_SIZE) {
 					set_msgf(sess->req, "Content Too Large (%lld)", ll);
 					goto error_return;
@@ -876,11 +920,8 @@ read_chunked(struct http_session *sess, FILE *out)
 		line = recv_line(sess, 5000, NULL);
 		if (line == NULL)
 			goto error_return;
-		errno = 0;
-		unsigned long chunk_size = strtoul(line, NULL, 16);
-		if (chunk_size == 0) {
-			if (errno == 0)
-				break;
+		unsigned long chunk_size;
+		if (!paranoid_strtoul(line, NULL, 16, &chunk_size)) {
 			set_msgf(sess->req, "strtoul() failure %d", errno);
 			goto error_return;
 		}