Get dexter building/running on OS/390 (same changes as those

made to mpmt_pthread previously):
. don't include netinet/tcp.h if you ain't got it
. handle OS/390 flavors of pthread_sigmask() and
  pthread_attr_setdetachstate()


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@85244 13f79535-47bb-0310-9956-ffa450edef68