Add support for an exports.db to mountd

[pen_lysator.liu.se]

Summary

Peter Eriksson submitted this patch, which adds support for a
database form of the exports file (as an alternative to the
exports(5) flat file). He has a patch for ZFS that generates
this database file format and it saves time when ZFS exports
a large number of file systems.

The current patch first looks for a file with the ".db" suffix and
uses that file instead of one without the ".db" suffix, if found.
(I am not 1000% sure if this semantic is appropriate vs requiring
the "exports.db" file name being an argument, but it avoids any
changes being needed for /etc/rc.d/mountd and avoids any
POLA, since behaviour remains the same unless "<file>.db" exists.

Although this is meant to be used with a patched version of ZFS,
the database can be generated manually via:
"makemap -f btree <file>.db < <file>"

A manual update will be done as a separate commit.

Test Plan

I have tested it with/without databases created manually
from exports(5) by makemap. Peter has tested a previous
version of the patch in his production environment.

[dan.mcgregor_usask.ca]

Neat. I should have looked at Phabricator before I created D42432. I think they should be considered together. And the patched version of zfs (I haven't looked, of course), should also consider special characters in mount point names if it doesn't.

[rmacklem]

Update the patch to include the recent changes for
handling blanks/tabs in directory paths.

Hopefully Peter can confirm that this version of the
patch works with an exports.db generated from a
patched ZFS?