ports/68281: add SPF support for milter-greylist

[Cyril Guibourg]

Hello Hajimu,

Hajimu UMEMOTO <ume at freebsd.org> writes:

>>Description:
> Though milter-greylist supports SPF but the port doesn't.

Yep, it was on my todo list :)

>>How-To-Repeat:
> building ports/mail/milter-greylist
>
>>Fix:
> Following patch added two options; WITH_SPF and WITH_SPF_ALT.
> WITH_SPF uses libspf and WITH_SPF_ALT uses libspf-alt.

I've been told that libspf_alt needs a reentrant version of libbind in
order to work fine. Are the available res_* libc routines thread safe ?

Thanks.