net: Improve and document SOCKS code

Summary:
Make the SOCKS code more consistent, and document the constants used.

This is a backport of Core PR11397

Depends on D1872

Test Plan:

make check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1873