Add -r/--run and -c/--cleanup, fix nsenter under root

-r/--run mounts an image, runs a command under bwrap in the
foreground (default /bin/sh, overridable via -- <command> [args...]),
then unmounts and cleans up when it exits. -c/--cleanup deletes a
layer and its ancestor chain from local storage (containers-storage
delete-layer, walking parents via `layer --json`), since -u only ever
unmounted.

bwrap needs to see the merged mount from inside the private namespace
containers-storage mount creates when running rootless; run_bwrap()
locates the live fuse-overlayfs process and runs bwrap via nsenter
into its namespaces. When running as root no such namespace exists
(containers-storage doesn't need to reexec for privilege), so nsenter
fails with EINVAL; detect geteuid() == 0 and skip it automatically
there. -n/--no-nsenter forces it off manually for any other case.

process.cpp gains run_process_foreground() (inherited stdio, for the
interactive bwrap run) and the relocated find_in_path(), now shared
with bwrap.cpp's nsenter lookup.

Also: meson test only ran -m, leaking a layer on every run; it now
runs tests/run_test.py, which drives mount -> umount -> cleanup and
fails if any step does.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

meson.build

@@ -27,4 +27,4 @@ fixture_tar = custom_target('oci-fixture',
     output : 'fixture.tar',
     command : [find_program('python3'), files('tests/gen_fixture.py'), '@OUTPUT@'])
 
-test('test', slocker_lite, args : ['-m', fixture_tar])
+test('test', find_program('python3'), args : [files('tests/run_test.py'), slocker_lite, fixture_tar])