mail/fetchmail{,conf}: Update 6.6.3 => 6.6.4
News:
https://gitlab.com/fetchmail/fetchmail/-/blob/6.6.4/NEWS
While here remove GNU_CONFIGURE_MANPREFIX - this is default value.
PR: 295178
Sponsored by: UNIS Labs
(cherry picked from commit 381f924c7dd2322bb2803799ffa03027356c2a9b)