Cleanup the "export PDU" common code.

exp_pdu_file_open() isn't used outside ui/tap_export_pdu.c; make it
static.

do_export_pdu() isn't a tap routine, it's called *from* tap routines, so
its last argument doesn't need to be a generic pointer; its last
argument must be a pointer to an exp_pdu_t, so declare it as such.

Clean up comments while we're at it.

Change-Id: Iab51b54a0c272052d7876110a095f2fff66fa2c1
Reviewed-on: https://code.wireshark.org/review/3060
Reviewed-by: Guy Harris <guy@alum.mit.edu>