h5edit

Bug fix:
When an attribute of the root group (e.g. /attr2 ) is modified,
the command fails to open the attached object ("/") because the
code tried to open it by H5Dopen() first.

Solution:
Changed modify to use H5Oopen/H5Oclose to open/close all dataset
or group objects. This not only fix the bug, it also simplifies
the code in the routine.

Tested: koala.

src/modify_cmd.c:
   code simplified for the solution above.

test/testh5edit.sh:
test/testfiles/modify-atts12-dset111.ref:
   Added the "MODIFY /attr2 ..." test to verify the solution.
   Changed the expect output too.

src/parse.y:
src/parse.c:
   Cleaned up a compiler warning message.