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>

passt.h

@@ -85,13 +85,15 @@ enum passt_modes {
 /**
  * struct ctx - Execution context
  * @mode:		Operation mode, qemu/UNIX domain socket or namespace/tap
- * @pasta_pid:		Target PID of namespace for pasta mode
  * @debug:		Enable debug mode
  * @quiet:		Don't print informational messages
  * @foreground:		Run in foreground, don't log to stderr by default
  * @stderr:		Force logging to stderr
  * @sock_path:		Path for UNIX domain socket
  * @pcap:		Path for packet capture file
+ * @pasta_netns_fd:	File descriptor for network namespace in pasta mode
+ * @pasta_userns_fd:	File descriptor for user namespace in pasta mode
+ * @netns_only:		In pasta mode, don't join or create a user namespace
  * @epollfd:		File descriptor for epoll instance
  * @fd_tap_listen:	File descriptor for listening AF_UNIX socket, if any
  * @fd_tap:		File descriptor for AF_UNIX socket or tuntap device
@@ -131,7 +133,6 @@ enum passt_modes {
  */
 struct ctx {
 	enum passt_modes mode;
-	int pasta_pid;
 	int debug;
 	int quiet;
 	int foreground;
@@ -139,6 +140,10 @@ struct ctx {
 	char sock_path[UNIX_PATH_MAX];
 	char pcap[PATH_MAX];
 
+	int pasta_netns_fd;
+	int pasta_userns_fd;
+	int netns_only;
+
 	int epollfd;
 	int fd_tap_listen;
 	int fd_tap;