Normalize HAS_* style configuration

[chrysn]

• HAS_RECVERR sits in socknumbers
• has_reuse_port sits in defaults

Both should be in defaults.

Granted, for has_reuse_port there may be a case for overriding it while there is none of HAS_RECVERR, but still there should be consistency. The detection method can stay or be replaced by sys.platform == 'linux', given that this is a custom Linux extension.