Save folks a few keystrokes. There is a command which already has a '-u'
option, document-export, but it doesn't support --user so there should
be no conflict. However '-s' is used by the info command among others,
so we can't use that for --system.
If a runtime is installed explicitly rather than as a dependency, pin it
so it doesn't get automatically removed when unused runtimes are being
removed. We do this because the runtime might be installed for
development or other uses.
This commit also rearranges some code in the mask and pin commands, and
adds a unit test.
As discussed here [1], we want a way to mark runtimes to be kept even
when they are unused by any apps and we are removing such runtimes.
Currently this is a command that can be run manually; a subsequent
commit will pin runtimes automatically if they are installed
independently of any app.
A unit test is included.
[1] https://github.com/flatpak/flatpak/issues/2639#issuecomment-662311756
