Disable buffered syscalls for tracess when they block SIGSYS.

To do so, the rr tracer has to track the blocked-signal set and detect
when SIGSYS-blockedness changes.  Buffered syscalls are disabled by
flipping off the TCB guard register, which makes it simpler to handle
task creation and syscallbuf initialization in the presence of a
blocked SIGSYS.

Resolves #731.