* fork.cc (lock_signals): Move to sigproc.h.

* fork.cc (lock_signals): Move to sigproc.h.
(lock_pthread): Ditto.
(hold_everything): Ditto.
(frok::parent): Call myself.prefork() just before calling CreateProcess.  Call
myself.postfork () on function exit.
* pinfo.cc (pinfo::pending_rd_proc_pipe): Define.
(pinfo::pending_wr_proc_pipe): Ditto.
(_pinfo::dup_proc_pipe): Delete.
(pinfo::wait): Move pipe creation into pinfo::prefork.  Set pipe variables from
pending_*.
(_pinfo::sync_proc_pipe): Delete.
(_pinfo::proc_pipe_owner): Ditto.
(pinfo::prefork): Define new function.
(pinfo::postfork): Ditto.
(pinfo::postexec): Ditto.
(_pinfo::alert_parent): Remove obsolete call to sync_proc_pipe.
(_pinfo::dup_proc_pipe): Delete declaration.
(_pinfo::sync_proc_pipe): Ditto.
(pinfo::pending_rd_proc_pipe): Declare.
(pinfo::pending_wr_proc_pipe): Ditto.
(pinfo::prefork): Declare new function.
(pinfo::postfork): Ditto.
(pinfo::postexec): Ditto.
(pinfo::wr_proc_pipe): Define new wrapper function.
* sigproc.h: Include "sync.h".  Move locking functions from fork to here.
* spawn.cc (child_info_spawn::worker): Delete now-unneeded requirement to
record orig_wr_proc_pipe.  Call hold_everything prior to doing anything.  Call
myself.prefork() if spawning.  Replace wr_proc_pipe synchronization with call
to myself.postexec().  Call myself.postfork() if not execing.
* sync.h: Replace #ifdef wrapper with "#pragma once".