io/FileDescriptor: fold CreateSignalFD() into the SignalFD constructor

src/io/FileDescriptor.cxx

@@ -236,21 +236,6 @@ FileDescriptor::CheckDuplicate(FileDescriptor new_fd) const noexcept
 
 #endif
 
-#ifdef __linux__
-
-bool
-FileDescriptor::CreateSignalFD(const sigset_t *mask) noexcept
-{
-	int new_fd = ::signalfd(fd, mask, SFD_NONBLOCK|SFD_CLOEXEC);
-	if (new_fd < 0)
-		return false;
-
-	fd = new_fd;
-	return true;
-}
-
-#endif
-
 bool
 FileDescriptor::Rewind() const noexcept
 {

src/io/FileDescriptor.hxx

@@ -9,10 +9,6 @@
 #include <unistd.h>
 #include <sys/types.h>
 
-#ifdef __linux__
-#include <csignal>
-#endif
-
 #ifdef _WIN32
 #include <wchar.h>
 #endif
@@ -180,10 +176,6 @@ public:
 	bool CheckDuplicate(FileDescriptor new_fd) const noexcept;
 #endif
 
-#ifdef __linux__
-	bool CreateSignalFD(const sigset_t *mask) noexcept;
-#endif
-
 	/**
 	 * Close the file descriptor.  It should not be called on an
 	 * "undefined" object.  After this call, IsDefined() is guaranteed