Remove obsolete docstring portions

Obseleted due to 38908cc
Hotfix bug in _close_all_but
Remove redundant os.sysconf call

subprocess.py already does exactly that call to try to set MAXFD
Use inherited pipes for IPC instead of STDIO

The main advantage being that STDIO streams can be redirected
independently without much effort.

Furthermore, this is very slightly faster in terms of startup time and
transfer speed when compared to _multiprocessing.Connection.
Put Connection related code into _libmadx_rpc

There are no changes in the actual code.
Minor cleanups