daemon: enable -Werror and many gcc warnings when --enable-gcc-warnings

* daemon/m4/gnulib-cache.m4: Add two modules: manywarnings, warnings.
* daemon/configure.ac: Implement --enable-gcc-warnings, and selectively
disable a few warning options that are either not useful or that provoke
too many warnings for now.
Define and AC_SUBST WARN_CFLAGS and WERROR_CFLAGS.
* daemon/Makefile.am (guestfsd_CFLAGS): Use $(WARN_CFLAGS)
and $(WERROR_CFLAGS), rather than just -Wall.