err.h: dead once is dead enough

Per joerg's advice keep the __dead in front position as it is where
_Noreturn would go.

Add the {v,}{warn,err}c function flavors from FreeBSD

branches: 1.16.2; 1.16.8;
Retire varargs.h support. Move machine/stdarg.h logic into MI
sys/stdarg.h and expect compiler to provide proper builtins, defaulting
to the GCC interface. lint still has a special fallback.
Reduce abuse of _BSD_VA_LIST_ by defining __va_list by default and
derive va_list as required by standards.

Consistently use __printflike and __scanflike.

branches: 1.14.44;
de-__P -- the hack is long since useless. Discussed with christos,
matt, kleink, others. Approved by christos.

Move UCB-licensed code from 4-clause to 3-clause licence.

Patches provided by Joel Baker in PR 22270, verified by myself.

__Namespace__ __protection__.

new RCS ID format.

better attributes.

_VA_LIST_ -> _BSD_VA_LIST_

Change __attribute -> __attribute__. The former isn't turned off by
sys/cdefs.h when not using Gcc, the latter is.

Use GCC's format attribute (for bug detection with -Wformat).

trim _BSD off of _BSD_VA_LIST, so that we don't have to change all
standard headers (yet).

update to latest version

Change volatile qualifier to __dead, for when cc != gcc.

Add RCS identifiers.

s/__dead/volatile/

branches: 1.1.1;
oops