Add several new vector interfaces to ovecmath.c

  These include mods, clamp, abs, floor, trunc, round, ceil and mod.
mods stand for mod with "s"calar argument.

  A change was done to oparser.c to allow the, currently special symbols
abs, floor, trunc, round and ceil to be changed back to a symbol. A better
and more complete change is already in TODO, but for now only a small
"hack" was added to support the new ovecmath.c interfaces.