MEDIUM: cfgparse: warn when using user/group when built statically

MEDIUM: cfgparse: warn when using user/group when built statically

In issue #3013, an user observed a crash at startup of haproxy when
building statically and using the "user" global section.

This is a known problem of the glibc and the linker even warn about
this:

> warning: Using 'getgrnam' in statically linked applications requires at runtime the shared libraries from the glibc version used for linking
> warning: Using 'getpwnam' in statically linked applications requires at runtime the shared libraries from the glibc version used for linking

Let's emit a warning when using user/group in this case.