Fix for x11 hash rate & shares display in MPOS compatibility mode

In order for pool and worker hash rate and worker shares to calculate and display correctly in MPOS compatibility mode for x11 coins **only**, two changes from scrypt coins are necessary:

1. MPOS must have `target_bits` set to 24
2. share difficulty must be multiplied by 256

This pull request implements a config option for the second change listed above.