Support a CPUS=<n> argument for the `in-place' makefile target

Also, make `distro-build/drive-clients' use `#:j' configuration
for `PLT_SETUP_OPTIONS'.
Add `-j'/`--jobs' <n> argument to `raco pkg install', etc.

Also, thread `PLT_SETUP_OPTIONS' though server and client build
(so that a `#:j' configuration works as intended).
switch makefile's parallelism control to `JOB_OPTIONS'

Avoid potential collision with `PLT_SETUP_OPTIONS' as an environment
variable.