introduce uppercase macros SOCKERRNO, SET_SOCKERRNO(), ERRNO and SET_ERRNO()

making them available to any source code file which includes "setup.h".

Macro SOCKERRNO / SET_SOCKERRNO() returns / sets the *socket-related* errno
(or equivalent) on this platform to hide platform details to code using it.

Macro ERRNO / SET_ERRNO() returns / sets the NOT *socket-related* errno
(or equivalent) on this platform to hide platform details to code using it.