Enable shell-like syntax for extra_args from file

[robinmatz]

With the current implementation of x3270._process_args arguments from a file are split by whitespaces, e.g.

# argfile.txt
-charset french

becomes ["-charset", "french"].

There are, however, resources that allow whitespace between the arguments, like -xrm "wc3270.blankFill: true". On a command line, the whitespace can be retained using single or double quotes. This is currently not possible with the implementation of x3270._process_args.

I would like something like

# argfile.txt
-xrm "wc3270.blankFill: true"

to be interpreted as ["-xrm", "wc3270.blankFill: true"]