showlinenum : Add option to allow colons in path

@allow_colons_in_path [0,1] default: 0
Allow colons in path.

By default colons are still banned to guarantee proper parsing of this
script's output, however prior to this change there was no way to
override that.
showlinenum : Standardize the error message format

- Document output format in usage comment block.

- (FATAL) New. Send all fatal errors to this function.

- Print error messages to stderr instead of stdout.

- (get_bool) Fix missing exit code in fatal error during param check.

Prior to this change if get_bool() aborted during the parameter check
it did not pass an exit code. Now it calls FATAL() which handles that.