Stop using wshelper for SRV/TXT lookups

Stop using wshelper for SRV/TXT lookups

Add Windows-specific versions of k5_try_realm_txt_rr(),
k5_make_uri_query(), and krb5int_make_srv_query_realm(), bypassing
wshelper and using DnsQuery_UTF8() directly.  Windows does not
currently know how to decode URI records and has no interface to allow
the caller to do so, so disable URI lookups on Windows for now.

Add a comment that dnsglue.h is not used on Windows (since DnsQuery
parses out and decodes SRV and TXT records for us), and remove the
header conditionals from it.

ticket: 8687