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>

man/newgidmap.1.xml

@@ -116,6 +116,17 @@
     <para>
       Note that newgidmap may be used only once for a given process.
     </para>
+    <para>
+      Instead of an integer process id, the first argument may be
+      specified as <replaceable>fd:N</replaceable>, where the integer N
+      is the file descriptor number for the calling process's opened
+      file for <filename>/proc/[pid[</filename>.  In this case,
+      <command>newgidmap</command> will use
+      <refentrytitle>openat</refentrytitle><manvolnum>2</manvolnum>
+      to open the <filename>gid_map</filename> file under that
+      directory, avoiding a TOCTTOU in case the process exits and
+      the pid is immediately reused.
+    </para>
 
   </refsect1>
 

man/newuidmap.1.xml

@@ -116,6 +116,17 @@
     <para>
       Note that newuidmap may be used only once for a given process.
     </para>
+    <para>
+      Instead of an integer process id, the first argument may be
+      specified as <replaceable>fd:N</replaceable>, where the integer N
+      is the file descriptor number for the calling process's opened
+      file for <filename>/proc/[pid[</filename>.  In this case,
+      <command>newuidmap</command> will use
+      <refentrytitle>openat</refentrytitle><manvolnum>2</manvolnum>
+      to open the <filename>uid_map</filename> file under that
+      directory, avoiding a TOCTTOU in case the process exits and
+      the pid is immediately reused.
+    </para>
   </refsect1>
 
   <refsect1 id='options'>