Extend duration of shield for gsf abjuration

[sharkinajar]

SF/GSF abjuration with shield is supposed to make up for not having mage armor, yet the spell only lasts 1 turn/level compared to mage armor's 1 hour/level. It would be a big help if the spell was 2 turns/level for GSF abjuration; makes it last a bit longer and not expire in the middle of quests as much.

Also has some weirdness where there's a lingering +4 ac for a few rounds (not the full +8 ) and you can't reapply the spell properly while this is happening.