init: Reland "Terminate gracefully when CAP_SYS_BOOT is absent"

This change makes it possible for Android running in a container to
terminate cleanly instead of calling abort() when requested to shut
down.

Bug: 62388055
Test: `adb reboot` on bullhead causes no kernel panics
Test: `adb reboot` on a system without CAP_SYS_BOOT makes init terminate
       nicely

Change-Id: I36b2298610f5b4a2bf8b05103d04804883df2c88
diff --git a/init/capabilities.h b/init/capabilities.h
index ef507a6..ede85c3 100644
--- a/init/capabilities.h
+++ b/init/capabilities.h
@@ -15,15 +15,21 @@
 #ifndef _INIT_CAPABILITIES_H
 #define _INIT_CAPABILITIES_H
 
-#include <linux/capability.h>
+#include <sys/capability.h>
 
 #include <bitset>
 #include <string>
+#include <type_traits>
 
 namespace android {
 namespace init {
 
+struct CapDeleter {
+    void operator()(cap_t caps) const { cap_free(caps); }
+};
+
 using CapSet = std::bitset<CAP_LAST_CAP + 1>;
+using ScopedCaps = std::unique_ptr<std::remove_pointer<cap_t>::type, CapDeleter>;
 
 int LookupCap(const std::string& cap_name);
 bool CapAmbientSupported();