Added the necessary code to support
group renaming. The primary API
is 'nc_rename_grp(int grpid, const char* name)'.
No test cases provided yet.
This also required adding a rename_grp entry
to the dispatch tables.



git-svn-id: https://sub.unidata.ucar.edu/netcdf/trunk@3309 2e3060e4-1ed8-cf1f-9cc2-9586e844a556