Add P_CENTER mode flag for printing centered multline text buffers/strings

When used with putmsg(), centers the output based on the longest line.
When used with printfile() line at a time mode, centers each line.
When used with printfile() open/close mode, centers just the first line.

So it's a little weird and wonky.

src/sbbs3/putmsg.cpp

@@ -125,6 +125,13 @@ char sbbs_t::putmsgfrag(const char* buf, int& mode, unsigned org_cols, JSObject*
 				l += 9; // Skip "<NUL>WRAPOFF@"
 		}
 	}
+	if (mode & P_CENTER) {
+		size_t widest = widest_line(str + l);
+		if (widest < term->cols && term->column == 0) {
+			term->cursor_right((term->cols - widest) / 2);
+			mode |= P_INDENT;
+		}
+	}
 
 	while (l < len && (mode & P_NOABORT || !msgabort()) && online) {
 		switch (str[l]) {

src/sbbs3/sbbsdefs.h

@@ -701,6 +701,7 @@ typedef enum {                       // Values for xtrn_t.event
 #define P_INDENT    (1 << 19)     // Indent lines to current cursor column
 #define P_ATCODES   (1 << 20)     // Trusted @-codes in formatted string
 #define P_MODS      (1 << 21)     // Display from mods/text dir, if file is there
+#define P_CENTER    (1 << 22)     // Center the output based on widest line
 
 #define P_XATTR_SHIFT 20
 #define P_WILDCAT   (SM_WILDCAT  << P_XATTR_SHIFT)