I.e. do what git does.  I'm too lazy to keep the builtin help text up
to date :-)

Also add ‘--help’ to various commands that lacked it
(e.g. nix-collect-garbage).

Nix now requires SQLite and bzip2 to be pre-installed.  SQLite is
detected using pkg-config.  We required DBD::SQLite anyway, so
depending on SQLite is not a big problem.

The --with-bzip2, --with-openssl and --with-sqlite flags are gone.