Function: getwalltime
Section: programming/specific
C-Name: getwalltime
Prototype:
Help: getwalltime(): time (in milliseconds) since the UNIX Epoch.
Doc: returns the time (in milliseconds) elapsed since
 00:00:00 UTC Thursday 1, January 1970 (the Unix epoch).
 \bprog
 my (t = getwalltime());
 ...
 print("Time: ", getwalltime() - t);
 @eprog
