Also now permit interactivly running tests without explicitly setting
$srcdir. This now works if we are inside ./tests and fails, as before,
when we are in a different directory.
Detected by shellcheck via CodeFactor.io
omrelp for some time limited authentication modes to those
that were known. While this was OK, it prevented the easy
introduction of new auth modes into librel.
This has now been changed; omrelp now checks the validity of
the authmode directly via librelp by doing some librelp calls
upon processing the configuration.
Also, some tests have been updated to check this feature and
also ensure that the new librelp mode "certvalid" works
(if it is available).
