* fhandler.h: Use #pragma once rather than ifdef guards.
(fhandler_console::tc_getpgid): Return our pgid if we have never opened a
console.
* fork.cc: Rearrange includes to accommodate fhandler.h use of pinfo.h.
* sigproc.cc: Ditto.
* spawn.cc: Ditto.
(child_info_spawn::worker): Query myself->pgid rather than calling expensive
function.
* thread.h: Use #pragma once rather than ifdef guards.
* pinfo.h: Use #pragma once rather than ifdef guards.
(pinfo::remember): Don't define if sigproc.h has not been included.
(pinfo::reattach): Ditto.
* sigproc.h: Use #pragma once rather than ifdef guards. Use different test to
see if pinfo.h has been included.
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