Add a rust user-space Capability struct.

This is a thin wrapper around a capability ptr that releases the
capability when it is done and prevents copying/cloning it.

To get a copy a caller must explicitly use duplicate.

rust/lib/mammoth/src/cap.rs

@@ -0,0 +1,48 @@
+use crate::cap_syscall;
+use crate::zion::{z_cap_t, ZError};
+
+pub struct Capability {
+    cap: z_cap_t,
+}
+
+impl Capability {
+    pub fn take(cap: z_cap_t) -> Self {
+        Self { cap }
+    }
+
+    pub fn take_copy(cap: z_cap_t) -> Result<Self, ZError> {
+        Ok(Self::take(cap_syscall::cap_duplicate(
+            cap,
+            Self::PERMS_ALL,
+        )?))
+    }
+
+    pub fn raw(&self) -> z_cap_t {
+        self.cap
+    }
+
+    pub fn release(mut self) -> z_cap_t {
+        let cap = self.cap;
+        self.cap = 0;
+        cap
+    }
+
+    pub const PERMS_ALL: u64 = u64::MAX;
+    pub fn duplicate(&self, perm_mask: u64) -> Result<Self, ZError> {
+        Ok(Self::take(cap_syscall::cap_duplicate(self.cap, perm_mask)?))
+    }
+}
+
+impl Drop for Capability {
+    fn drop(&mut self) {
+        if self.cap != 0 {
+            if let Err(e) = cap_syscall::cap_release(self.cap) {
+                crate::debug!(
+                    "WARN: error during cap release for cap {:#x}: {:?}",
+                    self.cap,
+                    e
+                );
+            }
+        }
+    }
+}