Improved parsing of "charset" parameter in MIME Content-Type header.

In Issue #177, the reported problem message header was "Content-Type: text/plain; charset=utf-8; format=flowed"
The fact that the "charset" value was not quoted and not space delimited means the charset would have been parsed as "utf-8;", which would not provide an exact match (against "utf-8") in smb_msg_is_utf8() and thus the message body would not be considered to be utf-8 encoded.

The solution is to terminate the "charset" parameter value at the semicolon, if it exists, and the value was not quoted.

Also, for good measure, only search for " charset" or ";charset" to avoid false-positive parameter matches,  like "notcharset".

src/smblib/smbtxt.c

@@ -367,15 +367,22 @@ void SMBCALL smb_parse_content_type(const char* content_type, char** subtype, ch
 				*tp = 0;
 			}
 		}
-		if(charset != NULL && (p = strcasestr(p, "charset=")) != NULL) {
-			p += 8;
-			if(*p == '"')
+		if(charset != NULL && ((p = strcasestr(p, " charset=")) != NULL || (p = strcasestr(p, ";charset=")) != NULL)) {
+			BOOL quoted = FALSE;
+			p += 9;
+			if(*p == '"') {
+				quoted = TRUE;
 				p++;
+			}
 			char* tp = p;
 			FIND_WHITESPACE(tp);
 			*tp = 0;
 			tp = p;
+			if(quoted) {
 				FIND_CHAR(tp, '"');
+			} else {
+				FIND_CHAR(tp, ';');
+			}
 			*tp = 0;
 			*charset = strdup(p);
 		}