Remove bloated ptrace flags

I have had a think, and these flags are pointless. Since this is a
microkernel, there should always be exactly one way to do something, and
it should be the most flexible and simplest to implement. And you can
already just set breakpoints on the address provided by the signal
handler, and we'll give the entrypoint as an address to the fexec call.

src/flag.rs

@@ -200,12 +200,6 @@ bitflags! {
         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;