bunpen: bind the different PR_CAP* prctl syscalls

see 'man prctl' for additional calls, some of which were omitted because i don't expect to need them

pkgs/additional/bunpen/rt/ext/capabilities.ha

@@ -292,6 +292,9 @@ export fn no_new_privs() (void | rt::errno) = {
   rt::prctl(rt::PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0)?;
 };
 
+// set the Effective, Permitted, and Inheritable capability vectors.
+// this does NOT effect the Ambient or Bounding capability sets;
+//   see PR_CAP_AMBIENT and PR_CAPBSET_DROP for that.
 export fn capset(eff: caps, prm: caps, inh: caps) (void | rt::errno) = {
   let hdr = user_cap_header {
     version = _LINUX_CAPABILITY_VERSION_3,
@@ -320,3 +323,22 @@ export fn capset(eff: caps, prm: caps, inh: caps) (void | rt::errno) = {
   ];
   syscall(rt::SYS_capset, &hdr: uintptr, &data: uintptr)?;
 };
+
+// add the provided capability to the ambient set.
+// the capability must already be in the permitted and inheritable sets (see `capset`).
+// will fail if PR_CAP_AMBIENT_LOWER secure bit has been set.
+export fn cap_ambient_raise(c: cap) (void | rt::errno) = {
+  rt::prctl(rt::PR_CAP_AMBIENT, rt::PR_CAP_AMBIENT_RAISE, c: u64, 0, 0)?;
+};
+
+// remove the provided capability from the ambient set.
+export fn cap_ambient_lower(c: cap) (void | rt::errno) = {
+  rt::prctl(rt::PR_CAP_AMBIENT, rt::PR_CAP_AMBIENT_LOWER, c: u64, 0, 0)?;
+};
+
+// drop the provided capability from the capability bounding set.
+// the effect is to make it so this thread generally can't acquire that capability
+// (combine that with NO_NEW_PRIVS to be certain).
+export fn capbset_drop(c: cap) (void | rt::errno) = {
+  rt::prctl(rt::PR_CAPBSET_DROP, c: u64, 0, 0, 0)?;
+};