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>

pcap.c

@@ -153,7 +153,7 @@ void pcapmm(struct mmsghdr *mmh, unsigned int vlen)
 /**
  * pcap_init() - Initialise pcap file
  * @c:		Execution context
- * @index:	pcap name index: passt instance number or pasta target pid
+ * @index:	pcap name index: passt instance number or pasta netns socket
  */
 void pcap_init(struct ctx *c, int index)
 {