[backport#14628] Rename misleading 'defaultPort' to 'http_port'

Summary:
4ed730802f9ec3d65477a29a318dd78216ef7085 scripted-diff: Rename misleading 'defaultPort' to 'http_port' (Murray Nesbitt)

Pull request description:

`defaultPort` in `HTTPBindAddresses()` is misleadingly named. `defaultPort ` suggests a constant, not something that might be overridden by `-rpcport`.

Tree-SHA512: f6ae8bdc2b4a4f503e44df9efdec32c854d2dede87714399f53791d50cce6bc41c46b01d1583cfc0e3e4777c244e1c74443fa39d9da50a45e53af265b74a17d1

This is a backport of Core PR14628

Test Plan:

ninja check

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5745