Fix python meterpreter subprocess deadlock and file descriptor leak

[adfoster-r7]

Depends on #651 being merged first

Fixes the following resource leak with subprocess.popen - caused by the stdout/stderr file descriptors not being closed:

$ python3 -m unittest discover -v ./tests
test_stdapi_net_config_get_interfaces (test_ext_server_stdapi.TestExtServerStdApi.test_stdapi_net_config_get_interfaces) ... <string>:2183: ResourceWarning: unclosed file <_io.BufferedReader name=3>
Object allocated at (most recent call last):
  File "/usr/local/Cellar/python@3.11/3.11.2_1/Frameworks/Python.framework/Versions/3.11/lib/python3.11/subprocess.py", lineno 1014
    self.stdout = io.open(c2pread, 'rb', bufsize)
<string>:2183: ResourceWarning: unclosed file <_io.BufferedReader name=5>
Object allocated at (most recent call last):
  File "/usr/local/Cellar/python@3.11/3.11.2_1/Frameworks/Python.framework/Versions/3.11/lib/python3.11/subprocess.py", lineno 1019
    self.stderr = io.open(errread, 'rb', bufsize)

Also fixes an edgecase with deadlocking when processes return large output:

Popen.wait(timeout=None)
Wait for child process to terminate. Set and return returncode attribute.

Note This will deadlock when using stdout=PIPE or stderr=PIPE and the child process generates enough output to a pipe such that it blocks waiting for the OS pipe buffer to accept more data. Use Popen.communicate() when using pipes to avoid that.

Relates to #569

[adfoster-r7]

Method manually tested with 2.4, as I couldn't get pip/mock installed with an older version of python

# cat test.py 
import subprocess

def get_process_output(args):
    proc_h = subprocess.Popen(args, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
    stdout, stderr = proc_h.communicate()

    if proc_h.wait():
        raise Exception(args[0] + ' exited with non-zero status')
    return str(stdout)


print(get_process_output(['ps', 'ax', '-w', '-o', 'pid,ppid,user,command']))

# python2.4 test.py 
  PID  PPID USER     COMMAND
    1     0 root     /bin/bash
 2157     1 root     python2.4 test.py
 2158  2157 root     ps ax -w -o pid,ppid,user,command