Revision 1994

Implement bin/update-db, to update the database with a single command.
Most processing is skipped if no input files have been modified.

lib/python/security_db.py (SchemaMismatch):
  New exception.
(DB):
  Handle schema versioning.
(DB.initSchema):
  Add subrelease column to source_packages and binary_packages.
  Set user_version.
  Remove stray commit.
(DB._parseFile):
  Return information to the caller if the file is unchanged.
(DB.readPackages):
  Move deletion code to callees.
(DB._readSourcePackages, DB._readBinaryPackages):
  Implement incremental updates.  Add subrelease.
  Need to invoke _clearVersions if any changes are made.
(DB.deleteBugs, DB.finishBugs):
  Moved into readBugs.
(DB.insertBugs):
  Rename ...
(DB.readBugs):
  ... to this one.  Implement incremental updates.
  Invoke _clearVersions if necessary.
(DB._clearVersions):
  Add.
(DB._updateVersions):
  Skip processing if _clearVersions has not been invoked.
(DB.getVersion, DB.releaseContainsPackage, DB._synthesizeReleases):
  Obsolete, remove.
(test):
  Update.

lib/python/bugs.py (CANFile, CVEFile):
  Split into two classes, which handle the differences between the two
  files.

bin/check-syntax:
  Update accordingly.

bin/update-db:
  New database update script.  Implements incremental updates.

Makefile:
  Remove references to bin/update-packages.  Simplify drastically.