Improve pdump file search and 'pdumper-stats'

Improve pdump file search and 'pdumper-stats'

* src/pdumper.c (pdumper_record_wd): New function.
(pdumper_load): Use xstrdup instead of strdup, as on
MS-Windows the latter uses the wrong heap.  Don't free a
NULL pointer.
* src/emacs.c (load_pdump): Support the use case where the
Emacs binary was renamed: look in exec-directory for the
pdump file whose base name is derived from the Emacs binary,
in addition to just emacs.pdmp.
(main): Call pdumper_record_wd to prepend CWD to the pdump
file name.
* src/fileio.c (file_name_absolute_p): Now extern.
* src/lisp.h (file_name_absolute_p): Add prototype.
* src/pdumper.h (pdumper_record_wd): Add prototype.

* doc/emacs/cmdargs.texi (Initial Options): Update the
documentation of where Emacs looks for the dump file.