run: add new --pipe option for including "systemd-run" commands in shell pipelines

In this mode, we'll directly connect stdin/stdout/stderr of the invoked
service with whatever systemd-run itself is invoked on. This allows
inclusion of "systemd-run" commands in shell pipelines, as unlike
"--pty" this means EOF of stdin/stdout/stderr are propagated
independently.

If --pty and --pipe are combined systemd-run will automatically pick the
right choice for the context it is invoked in, i.e. --pty when invoked
on a TTY, and --pipe otherwise.