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/event/SignalMonitor.cxx

@@ -38,13 +38,12 @@
 
 #include <algorithm>
 #include <cassert>
+#include <csignal>
 
 #ifdef USE_SIGNALFD
 #include <pthread.h>
 #endif
 
-#include <signal.h>
-
 class SignalMonitor final : private SocketMonitor {
 #ifdef USE_SIGNALFD
 	SignalFD fd;

src/system/FileDescriptor.hxx

@@ -38,7 +38,7 @@
 #include <sys/types.h>
 
 #ifdef __linux__
-#include <signal.h>
+#include <csignal>
 #endif
 
 #ifdef _WIN32

src/system/SignalFD.hxx

@@ -32,7 +32,7 @@
 
 #include "UniqueFileDescriptor.hxx"
 
-#include <signal.h>
+#include <csignal>
 
 /**
  * A class that wraps signalfd().

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");