pasta: Allow specifying paths and names of namespaces

Based on a patch from Giuseppe Scrivano, this adds the ability to:

- specify paths and names of target namespaces to join, instead of
  a PID, also for user namespaces, with --userns

- request to join or create a network namespace only, without
  entering or creating a user namespace, with --netns-only

- specify the base directory for netns mountpoints, with --nsrun-dir

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>
[sbrivio: reworked logic to actually join the given namespaces when
 they're not created, implemented --netns-only and --nsrun-dir,
 updated pasta demo script and man page]
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

test/demo/pasta

@@ -46,7 +46,7 @@ hostb	n
 sleep	10
 
 nl
-say	  without TARGET_PID, it will create a namespace.
+say	  without PID, it will create a namespace.
 sleep	3
 passt	cd __TEMPDIR__/passt
 passt	./pasta