* dcrt0.cc (sigthread::init): Reinstitute sigthread lock as a critical section.

* dcrt0.cc (sigthread::init): Reinstitute sigthread lock as a critical section.
(dll_crt0_1): Move sigthread lock initialization to earlier in startup.
* exceptions.cc (interrupt_on_return): Remove previous kludgy attempt to detect
an invalid frame.
(call_handler): Eliminate inner for loop.  Grab signal critical section lock
where appropriate.
* sigproc.cc (proc_subproc): Restore uid setting.
* sigproc.h (sigthread): Reinstitute sigthread lock as a critical section.
(sigframe): Grab the sigthread lock before clearing frame to avoid having the
signal thread use an invalid frame.