Drop --dbpath in favor of --datadir

[reynir]

Some commands take a --dbpath argument instead of assuming it lives in $BUILDER_WEB_DATADIR/builder.sqlite3 (passed by BUILDER_WEB_DATADIR environment variable or --datadir argument). This is inconsistent as some commands will take --datadir and look for the database file inside the datadir while other commands don't take --datadir as argument at all. I am thinking the flexibility of having the database file in other locations is not useful, so let's always assume it's in $BUILDER_WEB_DATADIR/builder.sqlite3