Some services require GUI access. Make qrexec-agent handling this, based
on per-service configuration, instead of forcing every caller to call
qubes.WaitForSession service first. This is especially important for
Disposable VMs, because those are destroyed after a single service call.
This needs to be done in qrexec-agent (instead of service script, or
qubes-rpc-multiplexer), because agent will behave differently depending
on GUI session being available or not. Namely, will use
qrexec-fork-server (so the process will be a child of session leader),
or will open new session.
Service configuration lives in /etc/qubes/rpc-config/SERVICE_NAME, can
can contain 'key=value' entries (no space around '=' allowed). Currently
the only settings supported is 'wait-for-session', with value either '0'
or '1'.
QubesOS/qubes-issues#2974
c8140375fa