Revert the reversion and go with implementation described in cgf-000017, with

some modifications.
* init.cc (dll_entry): Revert previous change.
* miscfuncs.cc: Include sigproc.h for exit_thread declaration.
* winsup.h (ExitThread): Define as 'exit_thread' to ensure no accidental use.
* sigproc.cc (exit_thread): New function.
(wait_sig): Handle __SIGTHREADEXIT case.  Don't just block rather than
returning from this function.
* sigproc.h (__SIGTHREADEXIT): New enum.
(exit_thread): Declare.
* sync.cc (muto::release): Accept a tls command-line argument.
* sync.h (muto::release): Accept a tls command-line parameter.  Default to
&_my_tls.
* cygerrno.h (__set_errno): Define as extern so that no function code is ever
emitted.
* cygserver_ipc.h (cygserver_ipc.h): Ditto.
* miscfuncs.h (transform_chars): Ditto.
* path.h (has_attribute): Ditto.
* security.h (privilege_luid): Ditto.
* winsup.h (flush_file_buffers): Ditto.

winsup/cygwin/cygserver_ipc.h

@@ -33,7 +33,7 @@ struct proc {
 
 #ifdef __INSIDE_CYGWIN__
 #include "sigproc.h"
-inline void
+extern inline void
 ipc_set_proc_info (proc &blk)
 {
   blk.cygpid = getpid ();