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