newuidmap and newgidmap: support passing pid as fd

Closes #635

newuidmap and newgidmap currently take an integner pid as
the first argument, determining the process id on which to
act.  Accept also "fd:N", where N must be an open file
descriptor to the /proc/pid directory for the process to
act upon.  This way, if you

exec 10</proc/99
newuidmap fd:10 100000 0 65536

and pid 99 dies and a new process happens to take pid 99 before
newuidmap happens to do its work, then since newuidmap will use
openat() using fd 10, it won't change the mapping for the new
process.

Example:

// terminal 1:
serge@jerom ~/src/nsexec$ ./nsexec -W -s 0 -S 0 -U
about to unshare with 10000000
Press any key to exec (I am 129176)

// terminal 2:
serge@jerom ~/src/shadow$ exec 10</proc/129176
serge@jerom ~/src/shadow$ sudo chown root src/newuidmap src/newgidmap
serge@jerom ~/src/shadow$ sudo chmod u+s src/newuidmap
serge@jerom ~/src/shadow$ sudo chmod u+s src/newgidmap
serge@jerom ~/src/shadow$ ./src/newuidmap fd:10 0 100000 10
serge@jerom ~/src/shadow$ ./src/newgidmap fd:10 0 100000 10

// Terminal 1:
uid=0(root) gid=0(root) groups=0(root)

Signed-off-by: Serge Hallyn <serge@hallyn.com>

lib/prototypes.h

@@ -160,6 +160,8 @@ extern int getlong (const char *numstr, /*@out@*/long int *result);
 
 /* get_pid.c */
 extern int get_pid (const char *pidstr, pid_t *pid);
+extern int get_pidfd_from_fd(const char *pidfdstr);
+extern int open_pidfd(const char *pidstr);
 
 /* getrange */
 extern int getrange (const char *range,