Fix laser discharging

Commit a6dae893d66319739e8dfe962f67285221eb9b91 introduced per-version
charge cost for firing mining lasers, but applies this in addition to
the old fixed cost which it was meant to replace.  Fix by removing the
application of the fixed cost.

The same commit did successfully change the check for a laser having
sufficient charge to fire, so that's based purely on the variable cost.
As a consequence, firing a laser that has just enough charge to cover the
variable cost could cause its charge to go negative.  (For example, by
fully charging a Mk1 laser and then firing it until it empties, resulting
in a charge of -400.)  It turned out that set_RE_wear handled that badly,
producing an over-100% wear value that would wrap to a *low* wear value,
leading to the laser's wear bar looking as if it's fully charged.

To protect against silly wear values, make set_RE_wear clamp the wear
value to avoid wrapping.  Handle specially the case of a fully-discharged
tool, where there was desirable wrapping to zero.
Merge pull request #3 from minetest-technic/master

update to latest master version