Closes [t:2612] Make separate override functions for use by toku_os_write(), toku_os_full_write(), toku_os_pwrite(), and toku_os_full_pwrite(). Modify src/tests/diskfull.c to use new override functions.

git-svn-id: file:///svn/toku/tokudb@20220 c7de825b-a66e-492c-adef-691d508d4ae1