Added Apple Xcode project files

Signed-off-by: Haru <haru@dotalux.com>
Changed plugin extension from .so to .dylib on OS X

- No functional changes. That's just the correct OS X file extension for
  dynamic libs.

Signed-off-by: Haru <haru@dotalux.com>
Nullpo cleanup

- Removed unused, nonportable nullpo checks with formatted message
  (related: eAthena r15245)
- Converted nullpo_chk to a macro, to make it easier on the llvm static
  analyzer.
- Added more details to the nullpo_info reports (related: eAthena
  r15246)
- Ensured that the nullpo check macros evaluate their pointer argument
  once and only once, so that it's safe to use with expressions that
  assign values or have side-effects.

Signed-off-by: Haru <haru@dotalux.com>
Added support for non-aborting assertions

- Added Assert_ret, Assert_retv, Assert_retb, Assert_retr, working
  similarly to the corresponding nullpo_ functions.
- Moved Assert-related macros to nullpo.h, since they share some
  functions.

Signed-off-by: Haru <haru@dotalux.com>
Fixed several compiler warnings

- Warnings detected thanks to Xcode's compiler settings (more strict by
  default) and clang, warnings mostly but not only related to data sizes
  on 64 bit systems, that were silenced until now by very lax compiler
  settings.
- This also decreases by a great deal the amount of warnings produced by
  MSVC in x64 mode (for the adventurous ones who tried that)
- Also fixed (or silenced in case of false positives) the potential
  issues pointed out by the (awesome) clang static analyzer.
- Patch co-produced with Ind, I'm merging and committing in his place!

Signed-off-by: Haru <haru@dotalux.com>
Increased warnings level in the unix build scripts

- They are now on par with the Xcode project (and similar to the MSVC
  ones) - this will make it easier to notice issues before committing in
  future.
- If this causes, on your system, warnings that we didn't detect, please
  let us know!

Signed-off-by: Haru <haru@dotalux.com>