Refactor LDAP DB option parsing code

krb5_ldap_open and krb5_ldap_create contain two large, almost
identical blocks of DB option processing code.  Factor it out into a
new function krb5_ldap_parse_db_params in ldap_misc.c, and simplify
the factored-out code.  Create a helper function to add server entries
and use it to simplify krb5_ldap_read_server_params as well as DB
option parsing.  Since the new DB option helper uses isspace instead
of isblank, we no longer require portability goop for isblank.
Add LDAP debug DB option

Add a DB option in the LDAP KDB module to turn on debugging messages.
Adapted from a patch by Zoran Pericic <zpericic@inet.hr>.

ticket: 7551 (new)
Fix prepend_err_str in LDAP KDB module

Use the oerr parameter to fetch the existing message.  Stop handling
oerr == 0, since no call sites were using it.  Free the old error
message before returning.
Note which release -x debug was added in