MFC: This is a selection of cleanups and bug fixes to the rpc.umntall
and mounttab code. It's not a direct MFC due to the TI-RPC changes
in -current, but it mostly corresponds to the following revisions:

rpc.umntall.c 1.5 - 1.8
mounttab.c 1.4, 1.5
mounttab.h 1.2
umount.c 1.27, 1.28

The main bug-fixes are:

• The `-e' option to rpc.umntall now works as expected.
• The logic for expiring mounttab entries is fixed.
• We don't block SIGINT/SIGQUIT while running.