Add function to calculate average result of dice roll

Part of response to ticket 379 (http://trac.rephial.org/ticket/379).

damrollavg(int num, int sides) calculates the average result
of a dice roll such as 2d6, so that the average damage of spells
and prayers may be calculated. The formula used is:

    ((num + (num * sides)) / 2)
Display average spell damage when browsing

Response to ticket 379 (http://trac.rephial.org/ticket/379).

Appends the average spell damage / range / duration etc. to the spell
info given in get_spell_info() using the formulas given with the
average dice roll (see damrollaverage() in src/z-rand.c).

e.g. A spell with damage 3d4 would have the info:
    dam 3d4 (7.5)

At present if the numbers involved are too large the information will
be clipped by the edge of the screen.