added cmd option to enable testing ACCESS_ONCE macro

option:
--enable-access-once

currently just a place holder, haven't implemented atcual functionality
added a class member variable isAddrTaken

indicates if a variable is address-taken or not
also set isAddrTaken when recording a addr-taken fact
added a member variable to indicate if a var wrapped by access_once

if isAccessOnce is true, then wrap the corresponding var
when dumping it.
set the probability of wrapping a var with ACCESS_ONCE macro

the default probability value is 20
randomly choose non-addr-taken vars for ACCESS_ONCE
commented out an unused var
output ACCESS_ONCE macro if --enable-access-once is passed
minor fix to parsing --enable-access-once
don't add ACCESS_ONCE to any var reference in a global's initializer
bug fix in recording addr-taken vars

handle cases where addr-taken operators appear in an array's initializer