New file unexec.h, the (simple) interface for unexec.

New file unexec.h, the (simple) interface for unexec.

* deps.mk (emacs.o, unexaix.o, unexcw.o, unexcoff.o, unexelf.o):
(unexhp9k800.o, unexmacosx.o, unexsol.o, unexw32.o):
Depend on unexec.h.
* emacs.c [!defined CANNOT_DUMP]: Include unexec.h.
* unexaix.c, unexcoff.c, unexcw.c, unexelf.c, unexhp9k800.c:
* unexmacosx.c, unexsol.c, unexw32.c: Include unexec.h.