replace signal.h with csignal.h

The former was deprecated with C++14. The standard says they are the same:

The contents of the header<csignal>are the same as the C standard library
header<signal.h>.

Signed-off-by: Rosen Penev <rosenp@gmail.com>

src/unix/Daemon.cxx

@@ -32,8 +32,8 @@
 #include <fcntl.h>
 
 #ifndef _WIN32
+#include <csignal>
 #include <sys/wait.h>
-#include <signal.h>
 #include <pwd.h>
 #include <grp.h>
 #endif

src/unix/SignalHandlers.cxx

@@ -28,7 +28,7 @@
 #include "system/Error.hxx"
 #include "util/Domain.hxx"
 
-#include <signal.h>
+#include <csignal>
 
 static constexpr Domain signal_handlers_domain("signal_handlers");