execute: Add new PassEnvironment= directive

execute: Add new PassEnvironment= directive

This directive allows passing environment variables from the system
manager to spawned services. Variables in the system manager can be set
inside a container by passing `--set-env=...` options to systemd-spawn.

Tested with an on-disk test.service unit. Tested using multiple variable
names on a single line, with an empty setting to clear the current list
of variables, with non-existing variables.

Tested using `systemd-run -p PassEnvironment=VARNAME` to confirm it
works with transient units.

Confirmed that `systemctl show` will display the PassEnvironment
settings.

Checked that man pages are generated correctly.

No regressions in `make check`.