Graceful handling of (temporary) read-only mounts during transaction

[dmnks]

Continuation of #3239, specifically see this comment:

Of course, the issue of underlying directory not getting updated goes for any (temporary) mounts we happen to touch during a transaction. This may call for stepping back a bit for an overall strategy (fancy expression for "I don't know" 😆 )

We know what mounts we cross paths with, and we technically even know which ones are readonly mounts. We just don't make a good use of that information: if we try to install something on such a filesystem we fail out with "not enough space" which is a bit confusing. Maybe we could automatically add readonly mounts to %_netsharedpath so they get skipped, but we shouldn't do that for an fs where we're actually trying to install to, only for the mountpoints.

And, that still leaves the question of what to do with any temporary mounts.

[pmatilai]

Related: #2782