Simplify tricky preprocessor tests in ff_check_visited()
Remove simpler cases of OOM error handling (after *alloc calls)

By simpler cases I mean cases where the OOM error is not expected to be handled
by the caller of the function that calls `alloc`, `lalloc`, `xrealloc`,
`xmalloc`, `alloc_clear`, and `lalloc_clear`.

These are the functions that:

 - Do not return an allocated buffer
 - Have OOM as the only error condition

I took note of the functions that expect the caller to handle the OOM error and
will go through them to check all the callers that may be handling OOM error in
future commits.

I'm ignoring eval.c and ex_.c in this series of commits. eval.c will soon be
obsolete and I will deal with ex_.c in later PRs.
Add klib.h to src/lib

As decided on #434 [1].

[1] https://github.com/neovim/neovim/issues/434#issuecomment-39111868
[2] https://github.com/attractivechaos/klib
Implement xcalloc and use it in klist.h (use xrealloc as well)

Bonus: implement lalloc_clear and alloc_clear using xcalloc
Reimplement the event queue in event.c using klist.h

 - Add a new macro to klist.h: kl_empty()

   The whole point of abstract data structures is to avoid reimplementing
   common actions. The emptiness test seems to be such an action.

 - Add a new function attribute to func_attr.h: FUNC_ATTR_UNUSED

   Some of the many functions created by the macros in klist.h may end up not
   being used. Unused functions cause compilation errors as we compile with
   -Werror. To mark those functions as possibly unused we can use the
   FUNC_ATTR_UNUSED now.

 - Pass `Event` by value

   `Event` is such a small struct that I don't think we should allocate heap space
   and pass it by reference. Let's use the stack and memory cache in our favor
   passing it by value.
Turn Event into a tagged union

If we ever need arbitrary data or more than very few bytes on `Events` we just
have to add a `void *` field in the `data` union.
Move functions from os/fs.c into path.c
Add headers
Add success constant
Make `os_get_absolute_path` static
Move functions dealing with executables
Use `os_getenv` instead of `getenv`
Unstatic os_is_absolute_path
Move unit tests to path
Move os_get_absolute_path tests to vim_FullName
Adapt tests to vim_FullName
Make os_get_absolute_path static, for good