py-compile: normalize error and help messages

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.