[Feature] Support /NOWAIT/ option when starting processes

[rsbeckerca]

GMAKE should support a /NOWAIT/ option for recipe commands. This option should report the completion code of the process start but not wait on the completion of the process.

It might be good to modify the list of children to support maintaining the list of background processes there as pHandles. This would be critical when collecting results during normal command execution in the case of a PROC_DEATH message arriving from a different process than the one being run.

We should ensure that behaviour for /NOWAIT/ is complaint with how GNU Make handles background process death.

[rsbeckerca]

Note that the code changes start here:

gmake/tandem.c

Line 120 in e7383ab

int launch_proc(char *argv[], char *envp[])

but will likely be more involved, including changes in

gmake/job.h

Line 87 in e7383ab

struct child

[rsbeckerca]

In order to do this, we would need the pHandle of the background process that was started, or the pName or both, particularly if the system assigns the name automatically. We would then need a push/pop mechanism for the names in order to track and use the name. A recipe might look like this:

something: source
        pathmon/name,nowait/
        pathcom /in source/ $(process name)
        pathcom $(process name);shutdown wait !
        $(process pop)

Thoughts. Maybe a different built-in name or syntax?