pkg/rootless: simplify reexec for container code

The code currently tried to avoid joining the userns from conmon
directly and rather joined to only read the pid file and then send this
back to use so we could join the userns. From the comment this was done
because we could not read the pid file. However this is no longer true
as of commit 49eb5af301 and file is no always owned by the real user.

This means we can just remove this special logic and join the namespace
directly there. A test has been added to check the rejoin logic with a
custom uidmapping.

Signed-off-by: Paul Holzinger <pholzing@redhat.com>

test/system/550-pause-process.bats

@@ -119,7 +119,7 @@ function _check_pause_process() {
 
     # First let's run a container in the background to keep the userns active
     local cname1=c1_$(random_string)
-    run_podman run -d --name $cname1 $IMAGE top
+    run_podman run -d --name $cname1 --uidmap 0:100:100 $IMAGE top
 
     run_podman unshare readlink /proc/self/ns/user
     userns="$output"
@@ -136,6 +136,9 @@ function _check_pause_process() {
 
     _test_sigproxy $cname2 $kidpid
 
+    # check pause process again
+    _check_pause_process
+
     # our container exits 0 so podman should too
     wait $kidpid || die "podman run exited $? instead of zero"