vim-patch:7.4.178

Problem:    The J command does not update '[ and '] marks. (William Gardner)
Solution:   Set the marks. (Christian Brabandt)

https://code.google.com/p/vim/source/detail?r=647e6bb15aa3f864eaf447fe77e3e3ae7e37b134
Implement `xstrdup` and `xstrndup`

Utility functions for copying strings to newly-allocated chunks. They either
succeed or exit the program.
Replace `strdup` calls by `xstrdup`