Userspace: Deal with select() returning EINTR on a signal interruption

Add a trivial CSafeSyscall template that calls a callback until it stops
returning EINTR, and use it everywhere we use select() now.

Thanks to Andreas for the suggestion of using a template parameter for
the syscall function to invoke.

Libraries/LibCore/CSyscallUtils.h

@@ -0,0 +1,25 @@
+#pragma once
+
+#include <errno.h>
+#include <unistd.h>
+#include <sys/types.h>
+#include <AK/StdLibExtras.h>
+
+namespace CSyscallUtils {
+
+template <typename Syscall, class... Args>
+inline int safe_syscall(Syscall syscall, Args&& ... args) {
+    for (;;) {
+        int sysret = syscall(forward<Args>(args)...);
+        if (sysret == -1) {
+            dbgprintf("CSafeSyscall: %d (%d: %s)\n", sysret, errno, strerror(errno));
+            if (errno == EINTR)
+                continue;
+            ASSERT_NOT_REACHED();
+        }
+        return sysret;
+    }
+}
+
+}
+