net: dedup MSG_NOSIGNAL and MSG_DONTWAIT definitions

Summary:
Deduplicate MSG_NOSIGNAL and MSG_DONTWAIT definitions from net.cpp
and netbase.cpp to compat.h where they can also be reused by other
code.

This is a backport of [[https://github.com/bitcoin/bitcoin/pull/20685 | core#20685]] [10/20]
https://github.com/bitcoin/bitcoin/pull/20685/commits/78fdfbea666201b25919dd67454eb04d6a34326f

Depends on D11018

Test Plan:
ninja all check-all

eviewers:

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11019