Add support for limit CPU and memory use via `systemd-run --user --scope -p CPUQuota=400% ...`

[HenrikBengtsson]

Add options --cpu-quota=<value>% and --memory-max=<size>, which, if CGroups v2 via systemd-run is supported, will launched rstudio-server via something like:

systemd-run --user --scope -p CPUQuota=400% -p MemoryMax=8G -p MemorySwapMax=8G rstudio-server

The defaults for these options should be configurable via environment variables, e.g. RSC_CPUQUOTA and RSC_MEMORY_MAX.

The environment variables will make it easy for sysadms to control the defaults.

[HenrikBengtsson]

Hmm... it looks like -p CPUQuota=400% only sets limits if run as root. It doesn't complain, there are no messages, but nothing is set.