Another round of Coverity cleanups, (minor) speedups, bug-fixes and test updates.

Corrected "BAIL" macros to avoid infinite loop when logging is disabled and an
    error occurs after an "exit:" label.

Corrected a dozen Coverity errors (mainly allocation issues, along with a few
    other things):
        711711, 711802, 711803, 711905, 970825, 996123, 996124, 1025787,
        1047274, 1130013, 1130014, 1139538

Refactored internal fill-value code to correctly handle string types, and
    especially to allow NULL pointers and null strings (ie. "") to be
    distinguished.  The code now avoids partially aliasing the two together
    (which only happened on the 'write' side of things and wasn't reflected on
    the 'read' side, adding to the previous confusion).

    Probably still weak on handling fill-values of variable-length and compound
    datatypes.

Refactored the recursive metadata reads a bit more, to process HDF5 named
    datatypes and datasets immediately, avoiding chewing up memory for those
    types of objects, etc.

Finished uncommenting and updating the nc_test4/tst_fills2.c code (as I'm
    proceeding alphabetically through the nc_test4 code files).