MFC 355600: Add a callout_func_t typedef for functions used with callout_*().

This typedef is the same as timeout_t except that it is in the callout
namespace and header.

Use this typedef in various places of the callout implementation that
were either using the raw type or timeout_t.

While here, add <sys/callout.h> to the manpage.

Revert r330897:

This was intended to be a non-functional change. It wasn't. The commit
message was thus wrong. In addition it broke arm, and merged crypto
related code.

Revert with prejudice.

This revert skips files touched in r316370 since that commit was since
MFCed. This revert also skips files that require $FreeBSD$ property
changes.

Thank you to those who helped me get out of this mess including but not
limited to gonzo, kevans, rgrimes.

Requested by: gjb (re)

MFC r314568 (by emaste):

kern_sig.c: ANSIfy and remove archaic register keyword

Sponsored by: The FreeBSD Foundation

MFC r318389 (by emaste):

Remove register keyword from sys/ and ANSIfy prototypes

A long long time ago the register keyword told the compiler to store
the corresponding variable in a CPU register, but it is not relevant
for any compiler used in the FreeBSD world today.

ANSIfy related prototypes while here.

Reviewed by: cem, jhb
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D10193

Partial merge of the SPDX changes

These changes are incomplete but are making it difficult
to determine what other changes can/should be merged.

No objections from: pfg

MFC r315289:
When draining a callout, don't clear CALLOUT_ACTIVE while it is running.

MFC 309148:
Permit timed sleeps for threads other than thread0 before timers are working.

The callout subsystem already handles early callouts and schedules
the first clock interrupt appropriately based on the currently pending
callouts. The one nit to fix was that callouts scheduled via C_HARDCLOCK
during early boot could fire too early once timers were enabled as the
per-CPU base time is always zero until timers are initialized. The change
in callout_when() handles this case by using the current uptime as the
base time of the callout during bootup if the per-CPU base time is zero.

MFC r303425:
Add callout_when(9).

MFC r303919:
Fix indentation.

Merge r302894, r303098: fix regression caused by r302350. Stopping a
not scheduled not running callout should return (-1).

PR: 210884
Approved by: re (gjb)