[Bug c/88270] -Wformat-XXX option for flagging %m

[eblake at redhat dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88270

--- Comment #5 from Eric Blake <eblake at redhat dot com> ---
For what it's worth, POSIX requires syslog() to support %m, and this is the
case even on BSD systems. So a new __attribute__((format(syslog, x, y)))
archetype would be an easy way of at least notating functions where %m must be
portable, even when flagging %m for printf.