Refactor: libpe_status: sort_rsc_priority() -> pe__cmp_rsc_priority()

Rename sort_rsc_priority() to pecmp_rsc_priority() to adhere to
current guidelines. Add a doc comment block. Move the declaration so
that pecmp_rsc_priority() is library-private.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>