Enable mod_cgid stderr handling via Unix socket fd transfer by

Enable mod_cgid stderr handling via Unix socket fd transfer by
default, rather than as an opt-in configure flag.  (This has been
tested in Fedora for >12 months without seeing any new problems)

* configure.in: Check for CMSG_DATA here, and define HAVE_FDPASSING
  and $ap_has_fdpassing if fd passing is supported.

* modules/generator/config5.m4,
  modules/generator/mod_cgid.c: Drop configure flag for mod_cgid
  fdpassing support, instead enable where possible by default.

* modules/proxy/config.m4: Rely on configure test for CMSG_DATA.

* .travis.yml: Switch test for cgid fdpassing for testing w/o
  CMSG_DATA.

Github: closes #172
PR: 54221

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1886912 13f79535-47bb-0310-9956-ffa450edef68