mount.nfs: add new string tokenizer facility

To quote the strtok(3) man page: "Avoid using these functions."

OK.  We've created our own.  The main reason for this is that strtok(3)
doesn't handle quoted delimiters at all.  We need to handle this:

   context="foo,bar"

where 'context' is a single mount option that sets a token string that
possibly uses the same delimiter that the mount command uses to separate
options (that is, a comma).

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Neil Brown <neilb@suse.de>