selinux: use grep to check api supports (#402)

This checks `selinux_state` and `current_sid` supports in a raw way.
Feels more reliable than the version checks.

Supersedes #401, fixes #280, fixes #400.

kernel/selinux/selinux.h

@@ -2,6 +2,11 @@
 #define __KSU_H_SELINUX
 
 #include "linux/types.h"
+#include "linux/version.h"
+
+#if (LINUX_VERSION_CODE >= KERNEL_VERSION(5, 10, 0)) || defined(KSU_COMPAT_HAS_SELINUX_STATE)
+#define KSU_COMPAT_USE_SELINUX_STATE
+#endif
 
 void setup_selinux();