Some Veriexec stuff that's been rotting in my tree for months.

Bug fixes:
- Fix crash reported by Scott Ellis on current-users@.

- Fix race conditions in enforcing the Veriexec rename and remove
policies. These are NOT security issues.

- Fix memory leak in rename handling when overwriting a monitored
file.

- Fix table deletion logic.

- Don't prevent query requests if not in learning mode.


KPI updates:
- fileassoc_table_run() now takes a cookie to pass to the callback.

- veriexec_table_add() was removed, it is now done internally. As a
result, there's no longer a need for VERIEXEC_TABLESIZE.

- veriexec_report() was removed, it is now internal.

- Perform sanity checks on the entry type, and enforce default type
in veriexec_file_add() rather than in veriexecctl.

- Add veriexec_flush(), used to delete all Veriexec tables, and
veriexec_dump(), used to fill an array with all Veriexec entries.


New features:
- Add a '-k' flag to veriexecctl, to keep the filenames in the kernel
database. This allows Veriexec to produce slightly more accurate
logs under certain circumstances. In the future, this can be either
replaced by vnode->pathname translation, or combined with it.

- Add a VERIEXEC_DUMP ioctl, to dump the entire Veriexec database.
This can be used to recover a database if the file was lost.
Example usage:

# veriexecctl dump > /etc/signatures

Note that only entries with the filename kept (that is, were loaded
with the '-k' flag) will be dumped.

Idea from Brett Lymn.

- Add a VERIEXEC_FLUSH ioctl, to delete all Veriexec entries. Sample
usage:

# veriexecctl flush

- Add a 'veriexec_flags' rc(8) variable, and make its default have
the '-k' flag. On systems using the default signatures file
(generaetd from running 'veriexecgen' with no arguments), this will
use additional 32kb of kernel memory on average.

- Add a '-e' flag to veriexecctl, to evaluate the fingerprint during
load. This is done automatically for files marked as 'untrusted'.


Misc. stuff:
- The code for veriexecctl was massively simplified as a result of
eliminating the need for VERIEXEC_TABLESIZE, and now uses a single
pass of the signatures file, making the loading somewhat faster.

- Lots of minor fixes found using the (still under development)
Veriexec regression testsuite.

- Some of the messages Veriexec prints were improved.

- Various documentation fixes.


All relevant man-pages were updated to reflect the above changes.

Binary compatibility with existing veriexecctl binaries is maintained.

Load Veriexec signatures after mountall. Use veriexec_strict and
veriexec_verbose to set strict level and verbose level in rc.conf.
Defaults are 0.

Revert previous commit; I'll find a better solution.

We need mountall here.

Run veriexec before securelevel and sysctl scripts. Suggested by Nino Dehne.

Rototill of the verified exec functionality.
* We now use hash tables instead of a list to store the in kernel
fingerprints.
* Fingerprint methods handling has been made more flexible, it is now
even simpler to add new methods.
* the loader no longer passes in magic numbers representing the
fingerprint method so veriexecctl is not longer kernel specific.
* fingerprint methods can be tailored out using options in the kernel
config file.
* more fingerprint methods added - rmd160, sha256/384/512
* veriexecctl can now report the fingerprint methods supported by the
running kernel.
* regularised the naming of some portions of veriexec.

branches: 1.3.2;
Add an _rc_subr_loaded variable, set to ":" by rc.subr. Scripts can use this
for a speedup by doing:
$_rc_subr_loaded . /etc/rc.subr

Missing rcvar. From Nicolas Joly in PR 24106.

Add startup script for verified exec.