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>

util.h

@@ -143,4 +143,4 @@ void bitmap_set(uint8_t *map, int bit);
 void bitmap_clear(uint8_t *map, int bit);
 int bitmap_isset(uint8_t *map, int bit);
 void procfs_scan_listen(char *name, uint8_t *map, uint8_t *exclude);
-int ns_enter(int target_pid);
+int ns_enter(struct ctx *c);