Update Seeder to use fsbridge::fopen() instead of fopen()
Updates seeder to use fsbridge::fopen().
Part of the process of cleaning up seeder code.
Confirm the files mentioned below are created and written to after the seeder runs:
dnsseed.dat dnseed.dump dnsstats.log
Reviewers: deadalnix, Fabien, jasonbcox, O1 Bitcoin ABC, #bitcoin_abc
Reviewed By: deadalnix, O1 Bitcoin ABC, #bitcoin_abc
Differential Revision: https://reviews.bitcoinabc.org/D4216