linux: wait for process before detach, and send SIGCONT

I had issues with the ptrace call failing because the process had not yet stopped
from SIGSTOP.
From this stackoverflow answer, it seems that you can use waitpid to
wait until the process is actually stopped. In python, this is exposed
as os.waitpid.

https://stackoverflow.com/questions/20510300/ptrace-detach-fails-after-ptrace-cont-with-errno-esrch#20525326

Additionally, the process was left frozen. I send a SIGCONT to continue
the process after the detach, so that it isn't left stopped.

mem_edit/linux.py

@@ -58,7 +58,9 @@ class Process(AbstractProcess):
 
     def close(self):
         os.kill(self.pid, signal.SIGSTOP)
+        os.waitpid(self.pid, 0)
         ptrace(ptrace_commands['PTRACE_DETACH'], self.pid, 0, 0)
+        os.kill(self.pid, signal.SIGCONT)
         self.pid = None
 
     def write_memory(self, base_address: int, write_buffer: ctypes_buffer_t):