The compiler doesn't tell us when we ignore a return value in C code

[briansmith]

• We should turn on the strictest "warn about any unused return values" warnings that every compiler (GCC, clang, MSVC) supports. Unfortunately, it seems like in general compilers don't have such a warning.
• If we can't make compilers warn about ignored return values without annotations, then we should add the annotations. In GCC this is __attribute__((warn_unused_result)). MSVC has one as well; see the documentation about SAL annotations. This should all abstracted using a macro to be defined in crypto/internal.h, named something like OPENSSL_WARN_UNUSED_RESULT.

Notes:

• There are likely a lot of small changes that are needed. Try to split the changes up into multiple commits.
• The GCC/clang work can be done separately from the MSVC work.

[briansmith]

This depends on #113, at least.