rfc23: support infinity as valid FSD

[grondo]

Problem: There are times when a duration must be encoded as infinity or unlimited, but FSD does not offer a good way to express that.

Support the string "infinity" as a special case in RFC 23.

[chu11]

The sentence somehow reads weird to me. I don't know why, I'll throw this out:

"As a special case, the string infinity SHALL be interpreted as a duration equivalent to the C99 macro INFINITY".

[grondo]

That works for me

[grondo]

Thanks @chu11! I'll set MWP.