src.opts.mk: force KERBEROS_SUPPORT off where KERBEROS forced off

Explicitly setting WITHOUT_KERBEROS implies WITHOUT_KERBEROS_SUPPORT,
but previously other cases that forced KERBEROS off (such as
WITHOUT_CRYPT) did not also set KERBEROS_SUPPORT off. Because the
_SUPPORT dependent options (KERBEROS/KERBEROS_SUPPORT) are processed
before other dependencies (CRYPT/KERBEROS) it's not easy to make this
happen automatically. Instead just explicitly set KERBEROS_SUPPORT
off where we set KERBEROS off.

Reported by: Michael Dexter's Build Option Survey run