vim-patch:7.4.486

Problem:    Check for writing to a yank register is wrong.
Solution:   Negate the check. (Zyx).  Also clean up the #ifdefs.

https://code.google.com/p/vim/source/detail?r=v7-4-486
Merge pull request #1730 from fwalch/vim-7.4.486

vim-patch:7.4.486