Returns a BIGSTR containing the expanded text of a PCB MACRO

Example:

	integer i,j, res
	j = PCBMAC("@Timelimit@")
	i = PCBMAC("@Timeused@")
	res = j-i
	println "You have ",res, " Minutes left"

PCB MACROS not supported:
@automore@ @beep@ @clreol@ @cls@ @delay@ @more@ @pause@ @poff@ @pon@ @pos@ @qoff@ @qon@ @wait@ @who@ @x@