security/cyrus-sasl2: separate sql plugins into security/cyrus-sasl2-sql
Fix circular-dependency problem happens after SASL/LDAP was added as
plugin to mysql57-client in r446496

Reviewed by: bapt, mat (mentor), ume (maintainer)
Approved by: mat (mentor), ume (maintainer)
MFH: 2017Q3
Differential Revision: https://reviews.freebsd.org/D11805