Replaced zaxisDefLtype()/zaxisInqLtype() by cdiDefKeyInt()/cdiInqKeyInt() with CDI_KEY_TYPEOFFIRSTFIXEDSURFACE.

* This way the MPI parts of CDI can be compiled without introducing
  an artificial MPI dependency for non-MPI parts.

* Also include namespaceNew and namespaceDelete in cdi.h.

* This also adds separate documentation, header, Fortran wrappers and
  pkg-config file for the cdi-pio part of the library

* Said generic can be replaced with an MPI-specific version, thus
  cleanly separating serialization of meta-data itself and MPI-
  communication.

* This results in an API change for CDI-pio initialization. Users are no longer
  required to specify the number of namespaces in advance.

* It became obsolete with the improvements to make_fint in r1955.