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 r250666:

Constify parameters, no functional or binary change.

MFC r235297, r235316:

Add a -v and -N option to kenv(1), so it can be more easily used in
scripts the way sysctl(8) is. The -N option, like in sysctl(8),
displays only the kenv names, not their values. The -v option prints an
individual kenv variable name with its value as name="value". This is
the inverse of sysctl(8)'s -n flag, since the default behaviour of
kenv(1) is already like sysctl(8) -n.

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

Approved by: re (implicit)

Add missing `void' for function without arguments.

Add a "-q" quiet flag to kenv so that warnings can be suppressed.

MFC after: 1 week

/*- or .\"- or #- to begin license clauses.

Various quibbles:
- Print a diagnostic if kdumpenv() fails. This can occur due to MAC
restrictions or lack of memory. Catch all kenv(2) failures as well.
- Just of the heck of it, DTRT if the kernel environment size changes
at the wrong time. The old code could fail silently or fail to
null-terminate a buffer if you got exceptionally unlucky.
- Sort and GC the #includes.

kenv(1) has been repo copied from usr.bin to bin.
Sometimes we need kenv(1) in /etc/rc.diskless*.

Consistently use FBSDID

Use the new functionalities provided by the kenv(2) syscall.

Reviewed by: peter

Deuglify the usage message by putting the program name after the
"usage:" string. This is how most (all?) other programs in the system
do it.

PR: 31596
Submitted by: Cyrille Lefevre <clefevre@citeweb.net>

Initial kenv(1) hack for dumping the kernel environment. This can be
used to extract modified boot hints to make loader(8)-time changes
"sticky". It tries to use \ style quoting so that it can be used directly
with foo.conf files. It can also extract specific variables.