Don’t needlessly convert to ‘unsigned’ in pdumper

Don’t needlessly convert to ‘unsigned’ in pdumper

* src/pdumper.c (PRIdDUMP_OFF): New macro.
(EMACS_INT_XDIGITS): New constant.
(struct dump_context): Use dump_off for relocation counts.
All uses changed.
(dump_queue_enqueue, dump_queue_dequeue, Fdump_emacs_portable):
Don’t assume counts fit in ‘unsigned’ or ‘unsigned long’.
Use EMACS_INT_XDIGITS instead of assuming it’s 16.