fc-cache: add a --root option

fc-cache: add a --root option

The point of the root support is to let fc-cache be usable on fs trees
other than just /.  This is useful to people cross-compiling, and for
non-root building (chroot won't work) for distro maintainers.  So now
we can tell fontconfig to search a specific root path to locate all of
its configs/fonts/caches/etc... without having to modify the configs.

We have to add a few new helper options for setting/getting the config
root, and then extend some of the existing API funcs to take a config
struct as an option.  This way the low level pieces know which paths
to actually search.

First we export the internal FcStat helper.  The fc-cache code uses
stat() directly, but we already have a FcStat helper to workaround
misc issues, so just use that one instead.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>