Add API to re-read the keyfile after writing it.
Usually, we would expect that whenever we serialize
something to disk, it can be read back exactly the same.
That is however not true for certificates, where we mangle
path and blobs while writing to file.
Anyway, extend the write-API to re-read what we just wrote.
The tests got extended to assert that whatever we write can
be read back the same.
Later, we want to reinject the reread connection into the settings
plugin again.
The tests were checking that the keyfiles had permission
0600 and there was a check-local target to prepare the permissons
of the tests.
That is inconvenient, and it is unexpected to have a check-local
target for such a case.
Also, the tests were not testing that keyfile reader would
bail out on invalid permissions. So just skip the check for
testing.
