Added new config option noautofillkeypool.

One of the most annoying things about trying to maintain wallet backups is the fact that every time you unlock a wallet the key pool gets automatically refilled, thus making any existing backups of the wallet obsolete. The user now has the choice to disable automatic refilling of the keypool by adding the following line to bitcoin.conf:

noautofillkeypool=1

This effectively disables all automatic key generation, requiring the user to manually run keypoolrefill to replenish the wallet when the key pool runs low. In addition, an optional parameter has been added to keypoolrefill to specify a backup destination so that backupwallet can be automatically called. This effectively ensures that any wallet backups contain the exact same set of keys.

If noautofillkeypool is not set in bitcoin.conf, the app defaults to its old behavior thus ensuring full backwards compatibility.