Improve documentation of pdumper; minor code cleanup

Improve documentation of pdumper; minor code cleanup

* src/emacs.c (usage_message): Add the --dump-file option.
(string_starts_with_p, find_argument): Functions removed; use
'argmatch' instead.
(PDUMP_FILE_ARG): Macro removed; use literal strings instead,
as with other command-line options.  Use HAVE_PDUMPER for cpp
conditionals which used PDUMP_FILE_ARG.
(load_pdump, main): Use 'argmatch' for "--dump-file" and
"--temacs" arguments, thus supporting the "-dump-file" and
"-temacs" variants, for consistency with other options.
(main): Remove the extra fatal error for using --dump-file in
unexec'ed Emacs: load_pdump does that anyway.
(standard_args): Add --dump-file and --temacs, with
appropriate priorities.

* etc/NEWS: Expand on the pdumper support.

* doc/emacs/cmdargs.texi (Initial Options): Document the
'--dump-file' command-line option.