Set socket options in child process after forking

Set socket options in child process after forking

Try to minimize the work done in the postmaster process for each
accepted connection, so that postmaster can quickly proceed with its
duties. These function calls are very fast so this doesn't make any
measurable performance difference in practice, but it's nice to have
all the socket options initialization code in one place for sake of
readability too. This also paves the way for an upcoming commit that
will move the initialization of the Port struct to the child process.

Discussion: https://www.postgresql.org/message-id/7a59b073-5b5b-151e-7ed3-8b01ff7ce9ef@iki.fi