Add PTRACE_STOP_SIGNAL_HANDLER

Not entirely a useful thing, one *could* also put a breakpoint on the
handler address... I'll need to think about this some more.

src/flag.rs

@@ -184,19 +184,43 @@ impl PartialAllocStrategy {
 
 bitflags! {
     pub struct PtraceFlags: u64 {
+        /// Stop before a syscall is handled. Send PTRACE_FLAG_IGNORE to not
+        /// handle the syscall.
         const PTRACE_STOP_PRE_SYSCALL = 0x0000_0000_0000_0001;
+        /// Stop after a syscall is handled.
         const PTRACE_STOP_POST_SYSCALL = 0x0000_0000_0000_0002;
+        /// Stop after exactly one instruction. TODO: This may not handle
+        /// fexec/signal boundaries. Should it?
         const PTRACE_STOP_SINGLESTEP = 0x0000_0000_0000_0004;
+        /// Stop before a signal is handled. Send PTRACE_FLAG_IGNORE to not
+        /// handle signal.
         const PTRACE_STOP_SIGNAL = 0x0000_0000_0000_0008;
+        /// Stop on a software breakpoint, such as the int3 instruction for
+        /// x86_64.
         const PTRACE_STOP_BREAKPOINT = 0x0000_0000_0000_0010;
+        /// Stop just before exiting for good.
         const PTRACE_STOP_EXIT = 0x0000_0000_0000_0020;
+        /// Stop before running a program switched to using `fexec`.
         const PTRACE_STOP_EXEC = 0x0000_0000_0000_0040;
+        /// Stop before a signal is handled in userspace. This will always
+        /// happen after a PTRACE_STOP_SIGNAL, if the `handler` argument wasn't
+        /// SIG_DFL or SIG_IGN. Send PTRACE_FLAG_IGNORE to not handle signal.
+        const PTRACE_STOP_SIGNAL_HANDLER = 0x0000_0000_0000_0080;
+
         const PTRACE_STOP_MASK = 0x0000_0000_0000_00FF;
 
+
+        /// Sent when a child is cloned, giving you the opportunity to trace it.
+        /// If you don't catch this, the child is started as normal.
         const PTRACE_EVENT_CLONE = 0x0000_0000_0000_0100;
+
         const PTRACE_EVENT_MASK = 0x0000_0000_0000_0F00;
 
+
+        /// Special meaning, depending on the event. Usually, when fired before
+        /// an action, it will skip performing that action.
         const PTRACE_FLAG_IGNORE = 0x0000_0000_0000_1000;
+
         const PTRACE_FLAG_MASK = 0x0000_0000_0000_F000;
     }
 }