hdf5_der

[svn-r24747] 1) Change names of public routines:
H5Ocork --> H5Odisable_mdc_flushes
H5Ouncork --> H5Oenable_mdc_flushes
H5Ois_corked --> H5Oare_mdc_flushes_disabled
2) test/cork.c: modified to use the new names
3) change name of use case example file: test/use_cork.c --> use_disable_mdc_flushes.c
4) test/cache.c test/cache_common.c: add comments
Tested on jam, koala, ostrich, platypus.

PENDING: 
a) code review 
b) hl/test/test_ld.c suddenly fails when using mpicc (via h5committest): need further investigation.