Issue reported by youkan700: for shopkeepers, taming via magic harp
behaved differently than taming via scroll or spell.
Make magic harp's taming be the same as [non-cursed] scroll of taming
and spell of charm monster: angry shopkeepers will be pacified (even
though they can't be tamed).
Also, add something I've been sitting on for ages: when taming magic
hits an already tame monster, give that monster a chance to become
tamer. Not significant for monsters that eat (unless being starved
for some reason) but matters for ones who don't eat. For tameness N
(which has a maximum of 20), if N is less than 10, have any taming
yield a 10-N out of 10 chance to increase the tameness by 1. So the
closer a pet is to becoming feral, the more likely for it to improve
tameness a little.
Closes#819
231bd75b7f