Get rid of write_headers global variable.

Have write_psml_preamble() and write_csv_preamble() take a capture_file *
as an argument, so they can print the column titles themselves, rather
than having to defer it to the routine that prints packet data.

Change-Id: Ifd1b7a13062be8ad46846315976922a752778153
Reviewed-on: https://code.wireshark.org/review/5438
Reviewed-by: Guy Harris <guy@alum.mit.edu>