MFC: r348590, r348591
Modify mountd so that it incrementally updates the kernel exports upon a reload.

Without this patch, mountd would delete/load all exports from the exports
file(s) when it receives a SIGHUP. This works fine for small exports file(s),
but can take several seconds to do when there are large numbers (10000+) of
exported file systems. Most of this time is spent doing the system calls
that delete/export each of these file systems. When the "-S" option
has been specified (the default these days), the nfsd threads are suspended
for several seconds while the reload is done.

This patch changes mountd so that it only does system calls for file systems
where the exports have been changed/added/deleted as compared to the exports
done for the previous load/reload of the exports file(s).
Basically, when SIGHUP is posted to mountd, it saves the exportlist structures
from the previous load and creates a new set of structures from the current
exports file(s). Then it compares the current with the previous and only does
system calls for cases that have been changed/added/deleted.
The nfsd threads do not need to be suspended until the comparison step is
being done. This results in a suspension period of milliseconds for a server
with 10000+ exported file systems.

There is some code using a LOGDEBUG() macro that allow runtime debugging
output via syslog(LOG_DEBUG,...) that can be enabled by creating a file
called /var/log/mountd.debug. This code is expected to be replaced with
code that uses dtrace by cy@ in the near future, once issues w.r.t. dtrace
in stable/12 have been resolved.

The patch should not change the usage of the exports file(s), but improves
the performance of reloading large exports file(s) where there are only a
small number of changes done to the file(s).

PR: 237860