This is a WIP commit to support Landlock; sandboxing similar to unveil()

This adds the capability to compile got-portable on FreeBSD.

The uuid.c source file was missing a trailing backslash.

Rather than having a toplevel EXTRA_DIST target in the toplevel Makefile.am file, move the relevant files to their own Makefile.am locations.

There is only one user which calls xrecallocarray: ibuf_realloc(). We already provide recallocarray() in the compat layer so ibuf_realloc() can simply call recallocarray() directly, as it does on OpenBSD. OK thomas.adam

The got_lib_privsep.h header expects the path to the libexec directory to be called GOT_LIBEXECDIR. At the moment it always falls back to using /usr/libexec. Fix what looks to be a typo.

This commit modifies the GoT main branch to be able to compile it under linux.