MFC boot tagging support: r337518, r337544-r337546, r337548,
r337579-r337580, r337952

This is equivalent to what's in head, except the default is an empty boot
tag string so that nothing gets output by default.

r337518:
kern: Add a BOOT_TAG marker at the beginning of boot dmesg

From the "newly licensed to drive" PR department, add a BOOT_TAG marker (by
default, --<<BOOT>>--, to the beginning of each boot's dmesg. This makes it
easier to do textproc magic to locate the start of each boot and, of
particular interest to some, the dmesg of the current boot.

The PR has a dmesg(8) component as well that I've opted not to include for
the moment- it was the more contentious part of this PR.

bde@ also made the statement that this boot tag should be written with an
ordinary printf, which I've- for the moment- declined to change about this
patch to keep it more transparent to observer of the boot process.

PR: 43434
Submitted by: dak <aurelien.nephtali@wanadoo.fr> (basically rewritten)

r337544:
msgbuf: Light detailing (const'ify and bool'itize)

r337545:
BOOT_TAG: Make a config(5) option, expose as sysctl and loader tunable

BOOT_TAG lived shortly in sys/msgbuf.h, but this wasn't necessarily great
for changing it or removing it. Move it into subr_prf.c and add options for
it to opt_printf.h.

One can specify both the BOOT_TAG and BOOT_TAG_SZ (really, size of the
buffer that holds the BOOT_TAG). We expose it as kern.boot_tag and also add
a loader tunable by the same name that we'll fetch upon initialization of
the msgbuf.

This allows for flexibility and also ensures that there's a consistent way
to figure out the boot tag of the running kernel, rather than relying on
headers to be in-sync.

Prodded super-super-lightly by: imp

r337546:
subr_prf: Use "sizeof current_boot_tag" instead

r337548:
subr_prf: style(9) the sizeof

Reported by: jkim, ian

r337579:
boot tagging: minor fixes

msgbufinit may be called multiple times as we initialize the msgbuf into a
progressively larger buffer. This doesn't happen as of now on head, but it
may happen in the future and we generally support this. As such, only print
the boot tag if we've just initialized the buffer for the first time.

The boot tag also now has a newline appended to it for better visibility,
and has been switched to a normal printf, by requesto f bde, after we've
denoted that the msgbuf is mapped.

r337580:
subr_prf: remove think-o that had returned to local patch

Reported by: cognet

r337952:
subr_prf: Don't write kern.boot_tag if it's empty

This change allows one to set kern.boot_tag="" and not get a blank line
preceding other boot messages. While this isn't super critical- blank lines
are easy to filter out both mentally and in processing dmesg later- it
allows for a mode of operation that matches previous behavior.

I intend to MFC this whole series to stable/11 by the end of the month with
boot_tag empty by default to make this effectively a nop in the stable
branch.