Provide ucontext to signal handlers

Add ucontext.h header, defining ucontext_t and mcontext_t types.

Provide sigaction sighandlers with a ucontext_t parameter, containing stack and
context information.

	* include/sys/ucontext.h : New header.
	* include/ucontext.h : Ditto.
	* exceptions.cc (call_signal_handler): Provide ucontext_t
	parameter to signal handler function.

Signed-off-by: Jon TURNEY <jon.turney@dronecode.org.uk>

winsup/cygwin/include/sys/ucontext.h

@@ -0,0 +1,26 @@
+/* ucontext.h
+
+   Copyright 2015 Red Hat, Inc.
+
+This file is part of Cygwin.
+
+This software is a copyrighted work licensed under the terms of the
+Cygwin license.  Please consult the file "CYGWIN_LICENSE" for
+details. */
+
+#ifndef _SYS_UCONTEXT_H_
+#define _SYS_UCONTEXT_H_
+
+#include <signal.h>
+
+typedef struct __mcontext mcontext_t;
+
+typedef struct __ucontext {
+	struct __ucontext *uc_link;
+	sigset_t	uc_sigmask;
+	stack_t	uc_stack;
+	mcontext_t	uc_mcontext;
+	unsigned long int	uc_flags;
+} ucontext_t;
+
+#endif /* !_SYS_UCONTEXT_H_ */