Add the certauth dbmatch module

Add the certauth dbmatch module

Add and enable the "dbmatch" builtin module.  Add the
pkinit_client_cert_match() and crypto_req_cert_matching_data() helper
functions.  Add dbmatch tests to t_pkinit.py.  Add documentation to
krb5_conf.rst, pkinit.rst, and kadmin_local.rst.

[ghudson@mit.edu: simplified code, edited docs]

ticket: 8562 (new)