[LinuxCNC/linuxcnc PR#299] gcodemodule: Make interp really close part program

For valid reasons (something to do with calling O-words from MDI),
the interpreter leaves the last file open even when close() is
called. However, open() actually does close the old file before
opening a new one.

This three step process ensures the old file is closed,
no new file is opened, and the interpreter’s internal state
is changed to “no file open”

@laurivosandi can you let me know if this fixes your issue?

Closes: #273