Extract jobserver implementation into OS-specific files.

* os.h, posixos.c, w32/w32os.c: New files implementing jobserver.
* job.c, job.h, main.c, makeint.h: Move content to new files.
* w32/include/sub_proc.h, w32/subproc/sub_proc.c: Ditto.
* Makefile.am: Build and package OS-specific files.
* build_w32.bat, make_msvc_net2003.vcproj, README.W32.template:
Update for new files, and clean up the build.
* POTFILES.in, maintMakefile, NMakefile.template: Ditto.
* w32/subproc/build.bat: Delete as unused.
19 files changed