Improve --enter-chroot to be more useful.

* --enter-chroot will now cause Haikuporter to enter a chroot for every phase,
  with build packages active and everything set up just like it would be for
  the real thing. This makes debugging of build/installation problems much
  easier.
Make checksum validation an integral part of the download phase.

* This avoids repeated checksum validations for ports that have already
  been unpacked.
Revert "Make checksum validation an integral part of the download phase."

* In case of validation failure, this causes the download to be repeated,
  which is much worse than redoing the validation.

This reverts commit 7ade5ca738dccd8cb5ff4947b72beb0db1ff6f6a.
Skip validation of checksum, 2nd try.

* Now successful validation of the checksum creates a corresponding
  flag file, so this shouldn't interfere with or depend on anything else.