*** WARNING: Segfaults on test9

Adds gvm_struct in gambit.h.in which currently contains fields related
to garbage collection (taken from global_state_struct) and it contains
pstate. Currently, it is created and managed EXACTLY like global_state_struct.

Creates ___GVM, ___GVMDECL, and ___SET_LOCAL_GVM. These macros are used
immediately before or after the corresponding GSTATE macro.

Redefines PSTATE so that it uses ___GVM.

 On branch para
  Changes to be committed:
    modified:   include/gambit.h.in
    modified:   lib/_kernel.scm
    modified:   lib/mem.c
    deleted:    lib/os_thread.c
    deleted:    lib/os_thread.h
    modified:   lib/setup.c