Modernize: use sigaction() whenever possible. Remove uses of common.

bsd_signal() may not be defined, and there is really no reason to even
try to use it if sigaction() is avaiable; using sigaction() guarantees
the semantics we really want.

Replace uses of common variables with explicit instantiation and
extern declarations in a header file.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>

common/signal.c

@@ -0,0 +1,42 @@
+/*
+ * signal.c
+ *
+ * Use sigaction() to simulate BSD signal()
+ */
+
+#include "config.h"
+
+#ifdef HAVE_SIGACTION
+
+sighandler_t tftp_signal(int signum, sighandler_t handler)
+{
+    struct sigaction action, oldaction;
+
+    memset(&action, 0, sizeof action);
+    action.sa_handler = handler;
+    sigemptyset(&action.sa_mask);
+    sigaddset(&action.sa_mask, signum);
+    action.sa_flags = SA_RESTART;
+
+    if (sigaction(signum, &action, &oldaction))
+	return SIG_ERR;
+
+    return oldaction.sa_handler;
+}
+
+#elif defined(HAVE_BSD_SIGNAL)
+
+sighandler_t tftp_signal(int signum, sighandler_t handler)
+{
+    return bsd_signal(signum, handler);
+}
+
+#else
+
+/* This is dangerous at best. Let's hope it works. */
+sighandler_t tftp_signal(int signum, sighandler_t handler)
+{
+    return signal(signum, handler);
+}
+
+#endif