Add os_init0 to allow for some really early OS inits.

On Linux, os_init can re-exec lisp to set up the correct personality.
Not normally a problem, but if any output happens before os_init is
called, the output appears to happen twice.  So add os_init0 to do
this early on, before any output.  This is a bit of a kludge.

lisp/lisp.c:
o Call os_init0 early in main.

lisp/Linux-os.c:
o Move the personality stuff from os_init to os_init0.

lisp/Darwin-os.c:
lisp/FreeBSD-os.c:
lisp/NetBSD-os.c:
lisp/OpenBSD-os.c:
lisp/hpux-os.c:
lisp/irix-os.c:
lisp/mach-os.c:
lisp/osf1-os.c:
lisp/solaris-os.c:
lisp/sunos-os.c:
o Add dummy implementation of os_init0.  These OSes don't (currently)
  need anything special.

lisp/os.h:
o Declare os_init0.