Revert "vim: treat _ as word boundary"

iskeyword-=_ breaks syntax highlighting, causing e.g. the "for" in
"list_for_each_entry" to be highlighted separately.
vim: add CamelCaseMotion

This is Kevin Le (bkad)'s fork of CamelCaseMotion, originally by Ingo
Karkat (inkarkat), taken at commit
ee138dc25f9b826af8b64fb17886fdf27a9267e9 of
https://github.com/bkad/CamelCaseMotion.