Fix spy stealing of future techs and of techs chosen by the spy

The renumbering of A_UNSET and A_FUTURE in r25394 broke stealing future techs
and letting the spy choose what tech to steal. This was caused by the argument
validation in diplomat_get_tech() assuming that A_UNSET and A_FUTURE is smaller
than A_LAST.

Rewrite the validation test so it don't make assumptions about what number
A_UNSET ("At Spy's Discretion") and A_FUTURE (future tech) has.

Reviewed by pepeto <pepeto>

See bug #22400

git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@25762 a0f10bec-cc02-0410-94fc-a9cfff90b4cd