Fix a couple of comments.
Fix an import path.
Correct all list/container loops that remove items.

It is not safe to remove an item from a container/list while iterating the
list without first saving the next pointer since removing the item nils
the internal list element's next pointer.
Handle another removal case missed in prev commit.

Also, the loops which only remove a single element and break or return
don't need the extra logic for iteration since they don't continue
iteration after removal.
Up some limits on !windows.

This is needed on OSX and possibly others.

ok davec
Add warning if there are no sync peers.

This is to help track down stalls that are being seen.
Update some comments and code consistency.
Move initial DNS seeding to own func.
Move DNS seed consts to same file that uses them.

Also, use multi constant define for the result.
Log transactions rejected from mempool as debug.

Most users won't care if a freestanding transaction is rejected from due
to a rule violation, but the information should still be available to
debug.
Misc cleanup.
Update findcheckpoint to support testnet and dbtype.
Add handling for mempool command.
Fixes for btcutil and btcscript api changes
Add updating instruction to README.md.
Go fmt and comments.
Switch to leveldb by default.

Also, add a warning if multiple block databases are detected.
Update to use latest btcwire invtype constants.
Remove extra msg in log message.
Add btcctl; currently a barebones utility to send commands
to btcd.