Test that LIBEXEC_NOREPLACE first copies the calling process's environment,
even if nothing is modified, but also test that LIBEXEC_NOCLOBBER does not
do this.

add LIBEXEC_RUN_ADD_SHARED_OUTPUT_FD
add LIBEXEC_RUN_ADD_SHARED_OUTPUT
add LIBEXEC_RUN_GET_STDOUT
add LIBEXEC_RUN_GET_STDERR
add LIBEXEC_RUN_SET_USER_TAG
add a way to create process substitutions in libexec_run
add a way to add a fork of the process into a pipeline

add man pages