Rename src/unexec.c => src/unexcoff.c.

 src/unexcoff.c: Renamed from unexec.c.
 src/deps.mk (unexcoff.o): Rename unexec.[co] => unexcoff.[co].
 configure.in (UNEXEC_OBJ): Rename unexec.o => unexcoff.o.
 admin/MAINTAINERS: Rename src/unexec.c => src/unexcoff.c.
 etc/AUTHORS: Rename unexec.o => unexcoff.o.
 etc/PROBLEMS: Rename unexec.o => unex*.o.
 lisp/emacs-lisp/find-gc.el (find-gc-source-files): Rename
 unexec.c => unexcoff.c.
 lisp/emacs-lisp/authors.el (authors-fixed-entries): Rename
 unexec.c => unexcoff.c.
 msdos/sed1v2.inp (UNEXEC_OBJ): Edit to unexcoff.o, due to renaming of
 unexec.c => unexcoff.c.