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.