Remove `lalloc_clear`

Use `xcalloc` instead. Change some local variables
to avoid casting.
vim-patch:7.4.236

Problem:    It's not that easy to check the Vim patch version.
Solution:   Make has("patch-7.4.123") work. (partly by Marc Weber)

https://code.google.com/p/vim/source/detail?r=a44087db72386d080e9da870d751daf498004be8
use fixed-width types to define u8char_T

The current approach was a bit ad-hoc.