Moves mem.c global variable inside a struct conveniently named
mem_instance (subject to change). Each use of those variables are done
through the ___MINSTANCE (subject to change) macro.

Add function to manage vms. These functions should be reworked to manage
pstate instead.

 On branch para
 Changes committed:
	modified:   lib/mem.c
	modified:   lib/os.c
	modified:   lib/os.h
	modified:   lib/setup.c
	modified:   lib/vm.c
	modified:   lib/vm.h
Makes mem_instance a vmstate field.
Corrects an omission in the last commit where gambit.h.in was not
correctly updated.

 On branch para
 Changes committed:
	modified:   include/gambit.h.in
	modified:   lib/mem.c
Make memory instance a member of vm state.
Adds next and prev fields to vmstates so they can be managed as a doubly
linked list.

 On branch para
 Changes to be committed:
	modified:   include/gambit.h.in
	modified:   lib/mem.c
	modified:   lib/os.c
	modified:   lib/setup.c
	modified:   lib/vm.c
	modified:   lib/vm.h