Fix handling of buffer size limit and update legacy interface

[dqwu]

This PR fixes two major issues with the pio_set_buffer_size_limit interface:

[Legacy Fortran library compatibility] Update the legacy Fortran interface to include an optional second argument to align with the newer interface used in E3SM/SCREAM, resolving confirmed build errors.

[Allow 0 as a valid buffer size limit] Update the handling of buffer size limit to allow 0 as a valid value and also fix calls to PnetCDF API ncmpi_buffer_attach.

Fixes #611