Function: getabstime
Section: programming/specific
C-Name: getabstime
Prototype: l
Help: getabstime(): time (in milliseconds) since startup.
Doc: returns the time (in milliseconds) elapsed since \kbd{gp} startup. This
 provides a reentrant version of \kbd{gettime}:
 \bprog
 my (t = getabstime());
 ...
 print("Time: ", getabstime() - t);
 @eprog
