Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

MFC r249951

According to devctl(4), clients must read events whole; they may not piece them
together from multiple reads(). It's as if /dev/devctl is a datagram device
instead of a stream device. However, devd's internal buffer was too small
(1025 bytes) to read an entire ereport.fs.zfs.checksum event (variable, up to
~1300 bytes). This commit enlarges the buffer to 8k.

Approved by: ken (mentor)

Copy head to stable/9 as part of 9.0-RELEASE release cycle.

Approved by: re (implicit)

Add the ability to match the on a media type of the device in question.

Submitted by: sam
Approved by: re (scottl)

Parse the ! lines that will soon be coming from the kernel. These are
a generalized notification mechanism for subsystems wishing to report
events.

Revieded by: njl

# The kernel side seems like it might be causing panics for me, but should
# be forthcoming shortly.

Create a new C++ header file called devd.hh which has all the class
definitions in it. Begin to document the classes that we use, and how
they interrelate (using comments that I can use with doxygen to
automatically generate docs with).

MFp4 (imp_freebsd branch): snapshot of devd work:
o improve parsing and lexing
o create data structures based on the parsed file now.
o Still need to rewrite main loop and add regex (still uses hard coded
devd-generic)
o minor man page updates.

# There should be one more commit before rc2

Approved by: re (blanket)

devd. A daemon that hooks into the kernel's /dev/devctl to produce
arbitrary commands when devices come and go in the device tree (which is
different than the /dev directory).

This is an initial version. Much of the planned power isn't here.
Instead of doing the full matching, we always run /etc/devd-generic.
/etc/devd.generic will go away at some point, I think.

I'm committing it in this early state so I can start getting feedback
from early adapters.

Approved by: re