Implement pdeathSignal handling in Runc (#431)

Add support for parent death signal (pdeathSignal) to ensure child
processes receive a signal when the parent process dies. This addresses
the FIXME comment in Runc.execute().

## Changes
- Add pdeathSignal field to exec_command_attrs C struct
- Implement prctl(PR_SET_PDEATHSIG) in child process handler (Linux
only)
- Expose pdeathSignal through Command.Attrs Swift API
- Wire up pdeathSignal in Runc.execute() to remove FIXME

## Implementation Details
The implementation uses Linux-specific prctl() to set the parent death
signal, ensuring proper cleanup when parent processes terminate. The
feature is conditionally compiled for Linux only, maintaining
compatibility with other platforms.

Sources/CShim/include/exec_command.h

@@ -38,6 +38,8 @@ struct exec_command_attrs {
   gid_t gid;
   /// signal mask for the child process
   int mask;
+  /// parent death signal (Linux only, 0 to disable)
+  int pdeathSignal;
 };
 
 void exec_command_attrs_init(struct exec_command_attrs *attrs);