fix: again select the faster proces starter when stdout/stderr are not
redirected
fix (perf under Linux): enable POSIX_SPAWN_USEFVORK correctly
reverting some of the changes for running ocamldep-postproc, because the
old way of running is now optimized
performance: avoiding to fork() for the frequent case of running the
pipeline ocamldep|ocamldep-postproc.
Merge branch 'posix_spawn' into perf-test

Conflicts:
	src/build/omake_build.ml
	src/shell/omake_shell_job.ml
added new testrun