Reapply fixes from pull request 214 (term-prompt)

[ksmith0]

Due to other merges the #214 pull request got clobbered. These changes from the term-prompt branch should be reincorporated.

When a command is entered the prompt is cleared until GetCommand is
called again. The KEY_UP and KEY_DOWN commands call clear_ and required
printing of the prompt after the clear_ as well. This permits long running commands such as adjust_offsets to indicate to the user that something is happening.
Prior to this the prompt would reappear immediately adfter the command was entered. If a command took some time to complete it appeared as if nothing happened and the user would retype the command to be surprised by the eventual output.