Include <sys/file.h> and [USG5] <fcntl.h> to define O_* macros.

Include <sys/file.h> and [USG5] <fcntl.h> to define O_* macros.
(O_RDONLY, O_RDWR): Define if undefined.
[emacs] (report_error): Use report_file_error so the user can tell what the
errno code was.
(unexec): Use O_RDONLY instead of explicit 0.
(make_hdr): Don't undo A_TEXT_OFFSET adjustment #ifndef NO_REMAP, since it
was never adjusted earlier #ifndef NO_REMAP.
(adjust_lnnoptrs): Use O_RDWR instead of explicit 2.