Introduce strbegins() macro and replace all open-coded instances

This test is done quite a bit across the tree, and the open-coded variants make it easy to have an
accidental mismatch between the length of the prefix being tested and the length actually passed to
strncmp().

This fixes an issue of that type comparing the server version against the prefix "u2.10", where the old
code used an incorrect length of 4.

source/mail.c

@@ -263,7 +263,7 @@ static  int	i = 0;
 						return NULL;
 
 					while (fgets(buffer, 254, mail))
-						if (!strncmp(MAIL_DELIMITER, buffer, 5))
+						if (strbegins(buffer, MAIL_DELIMITER))
 							count++;
 
 					fclose(mail);