Merge topic branch t/net into master

Merge topic branch t/net into master

A moderately sized series which contains a bunch of simple cleanups
for net.c and net.h.

* refs/heads/t/net:
  net: De-doxify static functions.
  net: Refer to correct man page in stringify_port().
  net: Pass true/false instead of 0/1 to makesock().
  net: Demote log level of error message in makesock().
  net: Rename para_connect_simple() -> para_connect().
  net: Make is_valid_ipv{4,6}_address() local to net.c.
  net: Remove IPPROTO_DCCP define.
  net: Make single-use macros local.
  net: Combine documentation of struct flowopts.
  net: Drop extern keyword of function declarations.