dissect: support mount options when going through mountfsd, requiring privileges...

RootImageOptions=/ExtensionImages=/MountImages= all support custom
mount options, but mountfsd does not support it. Add varlink
parameters to allow callers to specify mount options so that
those directives can work as expected. Require additional privs via
polkit.

diff --cc man/systemd.exec.xml
Simple merge

diff --cc src/core/namespace.c
Simple merge

diff --cc src/dissect/dissect.c
Simple merge

diff --cc src/nspawn/nspawn.c
Simple merge

diff --cc test/units/TEST-50-DISSECT.sh
Simple merge