Add 21 extra blank lines to the end of the 31-line DOOR.SYS file

Some doors/door kits actually require all 52 lines, but since they
(may) have problems with the values we populate on those additional
21 lines, just write them as blank when a program is configured for
the GAP (original) 31 line DOOR.SYS drop file. I doubt (and hope)
that no programs will care if their are more lines than they expect.

src/sbbs3/scfg/scfgxtrn.c

@@ -1510,12 +1510,16 @@ void xtrn_cfg(int section)
 						"  Solar Realms    DOORFILE.SR\n"
 						"  Synchronet      XTRN.DAT                      MODUSER.DAT\n"
 						"\n"
+						"`DOOR.SYS`\n"
 						"The drop file format compatible with the largest number of online\n"
 						"programs (e.g. door games) written for MS-DOS based BBSes is the\n"
 						"`DOOR.SYS` file format.  Synchronet supports both the original (GAP)\n"
-						"31-line DOOR.SYS file format and the extended 52-line format.\n"
-						"If you encounter door compatibility issues with the original 32-line\n"
-						"format, try using the 52-line format (or vice versa).\n"
+						"31-line DOOR.SYS file format and the extended (later) 52-line format.\n"
+						"The 31-line DOOR.SYS file created by Synchronet includes 21 extra blank\n"
+						"lines to accommodate software that expects more than 31 lines, but has\n"
+						"an issue with the contents of the additional lines as populated by SBBS.\n"
+						"If you encounter door compatibility issues with the original 31-line\n"
+						"format, try using the 52-line format instead (or vice versa).\n"
 					;
 					k=uifc.list(WIN_MID|WIN_ACT,0,0,0,&k,0
 						,"BBS Drop File Type",opt);

src/sbbs3/xtrn_sec.cpp

@@ -480,6 +480,12 @@ void sbbs_t::xtrndat(const char *name, const char *dropdir, uchar type, uint tle
 				,MIN(useron.posts, INT16_MAX));		/* 52: User message left */
 			lfexpand(str,misc);
 			fwrite(str,strlen(str),1,fp);
+		} else { // 31-line variant, followed by 21 blank lines
+			for(i = 0; i < 21; ++i) {
+				if(!(misc & XTRN_NATIVE))
+					fputc('\r', fp);
+				fputc('\n', fp);
+			}
 		}
 		fclose(fp);
 	}