add 2 new APIs: ncmpi_inq_num_rec_vars() and ncmpi_inq_num_fix_vars() that report the number of record and fixed-sized variables, respectively.

git-svn-id: https://svn.mcs.anl.gov/repos/parallel-netcdf/trunk@1776 c368ceb8-fd28-0410-a457-e46ee1d7cd4a