gotwebd: replace strftime(3) with asctime_r(3)

gotwebd: replace strftime(3) with asctime_r(3)

For the sake of consistency as discussed with stsp on IRC. All public
gotweb dates except the rss feed use asctime_r, as do tog, got, gotd,
and cvg. The remaining two template engine strftime instances are used
to produce rfc compliant rss and html datetime attribute formats.
Adapt the gotwebd patch test case to match the date format change.

ok stsp@