lsm-util: move detection of support of LSMs into a new lsm-util.[ch] helper

This makes the bpf LSM check generic, so that we can use it elsewhere.
it also drops the caching inside it, given that bpf-lsm code in PID1
will cache it a second time a stack frame further up when it checks for
various other bpf functionality.

src/shared/meson.build

@@ -105,6 +105,7 @@ shared_sources = files(
         'logs-show.c',
         'loop-util.c',
         'loopback-setup.c',
+        'lsm-util.c',
         'machine-id-setup.c',
         'machine-pool.c',
         'macvlan-util.c',