Support TB unit in config literals

[oberstet]

We are using "TB" as a unit in our PostgreSQL config (on box with 3TB memory):

oberstet@bvr-sql18:~/scm/3rdparty/pgtune$ sudo ./pgtune --type=dw /data/adr/postgresql.conf
Traceback (most recent call last):
  File "./pgtune", line 698, in <module>
    sys.exit(main(sys.argv))
  File "./pgtune", line 687, in main
    wizard_tune(config, options, settings)
  File "./pgtune", line 596, in wizard_tune
    config.update_setting(key, settings.show(key, value))
  File "./pgtune", line 275, in update_setting
    current = self.current_value(name)
  File "./pgtune", line 242, in current_value
    current = self.settings.parse(name, self.param_to_line[name].value())
  File "./pgtune", line 473, in parse
    return str(self.parse_int(name, value))
  File "./pgtune", line 456, in parse_int
    internal = int(value)
ValueError: invalid literal for int() with base 10: '2TB'