Adjust process launch --disable-aslr to take true/false value.

This change modifies the 'process launch' --disable-aslr option to take a boolean argument.  If the user directly specifies --disable-aslr {true,false}, that setting will control whether the process is launched with ASLR disabled accordingly.  In the event that the setting is not explicitly made on the process launch command line, then the value is retrieved from the target.disable-aslr setting (i.e. settings show target.disable-aslr).