rcfile: allow alternate line endings in nanorc files

When copy-pasting has resulted in a nanorc file with DOS line endings
(CR+LF), then silently ignore the carriage return, to avoid printing
an error message that partly overwrites itself.

This fulfills https://savannah.gnu.org/bugs/?57756.
Requested-by: Matthias Aßhauer <mha1993@live.de>

src/rcfile.c

@@ -1317,9 +1317,11 @@ void parse_rcfile(FILE *rcstream, bool just_syntax, bool intros_only)
 		if (just_syntax && !intros_only && lineno <= live_syntax->lineno)
 			continue;
 #endif
-		/* Strip the terminating newline, if any. */
+		/* Strip the terminating newline and possibly a carriage return. */
 		if (buffer[len - 1] == '\n')
-			buffer[len - 1] = '\0';
+			buffer[--len] = '\0';
+		if (buffer[len - 1] == '\r')
+			buffer[--len] = '\0';
 
 		ptr = buffer;
 		while (isblank((unsigned char)*ptr))