Merge Gntshr and Gnttab into one file

This enables the two modules to share types without their definitions
being exposed in the external interface.

Signed-off-by: John Else <john.else@citrix.com>
Replace "xenctrlext" with "xc"

Signed-off-by: John Else <john.else@citrix.com>
Tidy up the Gntshr interface and add comments

Signed-off-by: John Else <john.else@citrix.com>
Removed a few warnings.
Added ocamldoc headers.
Updated Eventch doc.
Use a boolean instead of RO | RW.
Local_mapping now local to Gnttab.
Merge pull request #17 from vbmithr/common-granttable-interface

Common granttable interface