this adds functions in conf.sh.common to create DS-style trust anchor
files. those functions are then used to create nearly all of the trust
anchors in the system tests.
there are a few exceptions:
- some tests in dnssec and mkeys rely on detection of unsupported
algorithms, which only works with key-style trust anchors, so those
are used for those tests in particular.
- the mirror test had a problem with the use of a CSK without a
SEP bit, which still needs addressing
in the future, some of these tests should be changed back to using
traditional trust anchors, so that both types will be exercised going
forward.
Reduce code duplication by replacing a code snippet repeated throughout
system tests using "trusted-keys" and/or "managed-keys" configuration
sections with calls to keyfile_to_{managed,trusted}_keys() helper
functions.
