Prepare hesokuri.main to implement some extra commands

This is also a clean-up, in order to maintain consistency between new and old
code.

- Functions that calculate a value based on environment variables and system
  properties have been changed to plain variables that are evaluated once at
  start-up. This prevents code from having to bind the function to a local
  variable when using it. This also means config-file is no longer testable, but
  making config-file a function made the -main function more complicated, and
  that function is NOT tested, so overall it was harming stability.
- Add a usage message.
- Rename port to web-port to avoid confusion.
- Used the Java system property user.home rather than the environment variable
  HOME. The latter generally does not have a value on Windows.

Symbols in this namespace are not, and should not be referenced by other
namespaces, so refactoring like this is not risky. Tried all new and existing
code paths to manually confirm functionality.
Readability and documentation adjustments

Prompted by comments in https://github.com/google/hesokuri/pull/13
Merge pull request #13 from matvore/usr-int

Prepare hesokuri.main to implement some extra commands