Log Message:
Cleaned up and refactored code.

The getopt and pkgver code have been extracted to separate packages to make
them reusable.

Several other functions have been moved to make the structure easier to
understand:

* dir.go and main.go have been moved to pkglint.go
* utility functions from pkglint.go have been moved to mkline.go

Now pkglint.go contains only high-level code.