Added `M-x org-resolve-clocks' command

Added `M-x org-resolve-clocks' command

(org-clock-resolve-clock): New function that resolves a clock to a
specific time, closing or resuming as need be, and possibly even
starting a new clock.

(org-clock-resolve): New function used by `org-resolve-clocks' that sets
up for the call to `org-clock-resolve-clock'.  It determines the time to
resolve to based on a single-character selection from the user to either
keep time, subtract away time or cancel the clock.

(org-resolve-clocks): New user command which resolves dangling clocks --
that is, open but not active -- anywhere in the file list returned by
`org-files-list'.

(org-clock-in): Automatically resolve dangling clocks whenever a user
clocks in.

(org-clock-cancel): If the user cancels the solely clock in a LOGBOOK,
remove the empty drawer.