Convert closesocket 'compat wrapper' to function in netbase

Simpler alternative to #4348.

The current setup with closesocket() is strange. It poses
as a compatibility wrapper but adds functionality.

Rename it and make it a documented utility function in netbase.

Code movement only, zero effect on the functionality.
Merge pull request #4504

43f510d Convert closesocket 'compat wrapper' to function in netbase (Wladimir J. van der Laan)