system/FileDescriptor: add openat() wrapper

src/system/FileDescriptor.cxx

@@ -76,6 +76,18 @@ FileDescriptor::IsSocket() const noexcept
 
 #endif
 
+#ifdef __linux
+
+bool
+FileDescriptor::Open(FileDescriptor dir, const char *pathname,
+		     int flags, mode_t mode) noexcept
+{
+	fd = ::openat(dir.Get(), pathname, flags | O_NOCTTY | O_CLOEXEC, mode);
+	return IsDefined();
+}
+
+#endif
+
 bool
 FileDescriptor::Open(const char *pathname, int flags, mode_t mode) noexcept
 {

src/system/FileDescriptor.hxx

@@ -116,6 +116,11 @@ public:
 		return FileDescriptor(-1);
 	}
 
+#ifdef __linux
+	bool Open(FileDescriptor dir, const char *pathname,
+		  int flags, mode_t mode=0666) noexcept;
+#endif
+
 	bool Open(const char *pathname, int flags, mode_t mode=0666) noexcept;
 
 #ifdef _WIN32