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