fix github pull request #144 - uswapwep vs shield

Fixes #144

While giving a new character his/her starting equipment, don't clear
alternate weapon when putting on a shield since that weapon should
still be available to swap with primary.  Affected knight's lance
and valkyrie's dagger.

Also, guard against wearing an initial shield if already wielding a
two-handed weapon, or wielding such a weapon if already wearing a
shield.  Both situations are hypothetical with current starting gear.

doc/fixes36.2

@@ -161,6 +161,8 @@ change default value for the 'autodescribe' option to 'on'
 Elbereth hypocrisy penalty doesn't apply if attacking a monster which isn't
 	frightened by Elbereth; normal scuffing of engravings still applies
 Elbereth hypocrisy penalty reduced when alignment is already low
+during character creation, don't unset alternate weapon when a shield gets
+	worn (was preventing knight from having lance set up as uswapwep)
 
 
 Fixes to Post-3.6.1 Problems that Were Exposed Via git Repository