Move files from src/ to src/nvim/.
- src/nvim/ becomes the new root dir for nvim executable sources.
- src/libnvim/ is planned to become root dir of the neovim library.

- Replace short_u with uint16_t (same size, should give no problems).
- When possible, remove explicit downcasts so that they are found when
  flag -Wconversion enabled.
- Remove short_u typedef.

Requested in #459.

reverting broad cosmetic/style change because:
- increases merge-conflicts
- increases overhead of merging upstream Vim patches
- reasons for change are ambiguous, so default to no change

Though correct (to my understanding), some of the casts introduced in
previous commits could lead to confusion, by casting only the first
expression component instead of the whole expression.

This fixes that, parenthesizing casted expressions, except where
operator precedence rules make it unneccesary.

Fix uses of plain "%ld" within sprintf():
- Replace "%ld" with "%" PRId64.
- Cast corresponding argument to (int64_t).

Removed modelines are better than modelines that differ from file to
file.

Problem:    Escaping a file name for shell commands can't be done without a
            function.
Solution:   Add the :S file name modifier.

https://code.google.com/p/vim/source/detail?r=40f18a1c1592c8b4047f6f2a413557f48a99c55f