Update Seeder to use fsbridge::fopen() instead of fopen()

Summary:
Updates seeder to use fsbridge::fopen().

Part of the process of cleaning up seeder code.

Test Plan:

make
bitcoin-seeder

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