build: Look for /usr/bin/gpgv2 vs /usr/bin/gpgv

For some reason, RHEL has gpgv, but Fedora doesn't.  We need to detect
which to use, since presumably Debian only has gpgv.
repo: Add API to provide xattrs

This will be used by rpm-ostree to provide SELinux security contexts,
without requiring us to actually label the disk.
Add SELinux support

The trees as shipped come with /usr/etc, which should just be labeled
as usr_t.  When we do a deployment, we need to relabel the copies of
the files we're making in /etc.

SELinux support is compile and runtime optional.