Support old-style flags even in the new flag system.

E.g., --compile-java-foo-bar anywhere on the cmd-line is the same
as ./pants compile.java --foo-bar.

This will make migration easier, as people are used to the old style,
and we may even want to keep this forever, if some users prefer
the verbose flags.

Testing Done:
Ran all the tests.

Reviewed at https://rbcommons.com/s/twitter/r/1093/
Tiny tweak to legacy option printing.

Also rewrote some comments for clarity.

Reviewed at https://rbcommons.com/s/twitter/r/1094/