Add attributes and methods to gdb.Inferior

Add attributes and methods to gdb.Inferior

This adds two new attributes and three new methods to gdb.Inferior.

The attributes let Python code see the command-line arguments and the
name of "main".  Argument setting is also supported.

The methods let Python code manipulate the inferior's environment
variables.

Reviewed-By: Eli Zaretskii <eliz@gnu.org>