allocating the thread context fails from Sactivate_thread. before
this change, the error was raised on the main thread, which resulted
in strange behavior at best. also added who argument to
S_create_thread_object to allow it to report either Sactivate_thread
or fork-thread led to the error.
externs.h, schsig.c, scheme.c, thread.c
original commit: 89691cee27ee7d5e9bffae530b6346e96f8cc7ad
