[RDY] Use portable format specifiers

I've given #490 a try.

What I've done is:
- Replace `%lld` and `%ld` within printf format string with `"%" PRId64`.
- Cast corresponding argument to (int64_t).
- In *.po files, use corresponding `%<PRId64>` facility.

I've used casting because from what I read at #490 and #296, I understood the
task of converting those arguments to real (int64_t) type is considered another
task, for which this one is a prerequisite.

For ease of review, I've subdivided commits by function invoked (sprintf,
vim_snprintf, etc.) and by localization (whether the format string gets
localized before invoking printf-like function). 

This passes all tests on my machine, but I'm on 64 bits.
This can fail on 32 bits, but only because of a last thing that remains
to be done: fixing the vim propietary implementation of snprintf/vsnprintf. This
is:

- Vim implements its own portable version of snprintf/vsnprintf. See 
  https://github.com/neovim/neovim/blob/master/src/message.c#L3086
- That implementation doesn't support 'll' length specifier. See 
  https://github.com/neovim/neovim/blob/master/src/message.c#L3104.
- Which is a bit weird, since the original code at
  http://www.ijs.si/software/snprintf/ does support 'll'.

So, we can:

- Fix vim own implementation of snprintf/vsnprintf.
- Remove it alltogether and use system version of those functions. It is
  supposed that, in C99 that we are targeting, those should be supported by all
  major compilers/platforms.

I'm not sure about Vim's motivation to adapt code from
[http://www.ijs.si/software/snprintf/]. In particular, why leaving out support
for 'll' if it was there? Is there some reason I don't know for that?
UPDATE: `vim_snprintf` adds `typval_T *tvs` parameter, used by eval machinery. See later comment on this.

Also, although system-level snprintf/vsnprintf are supposed to be available in
C99, I'm not sure about whether that's really the case or if it poses problems in
practice. I've found contradictory opinions in the web to that respect.

So, what do you think?