Allow unw_get_proc_name() to be called from signal-handler (for
cursors in the local address-space).
Clarify that unw_get_proc_info(), unw_get_proc_info_by_ip(),
unw_init_remote(), unw_resume(), and unw_step() are async-signal
safe only from the local address-space (we can't make any guarantees
beyond that because of the dependency of the per-address-space
callback routines, which may have all kinds of restrictions).

BKrev: 406a766eg-xRWnoMgbI9aFyk1xBWXA