py-compile: normalize error and help messages * lib/py-compile: Now error messages do not begin with a capital letter, nor end with a period, as per GNU standards. Prepend the error messages with the name of the script, not with its path. When an invalid usage is recognized, always display the customary message "Try `py-compile --help' ..." on a line of its own. ($me): New variable, containing the name of the program, i.e., `py-compile'. Use it throughout. (usage_error): New function, used to display error messages about invalid usage. * tests/py-compile-usage.test: Extend and tighten accordingly.