git__getenv: utf-8 aware env reader

Introduce `git__getenv` which is a UTF-8 aware `getenv` everywhere.
Make `cl_getenv` use this to keep consistent memory handling around
return values (free everywhere, as opposed to only some platforms).

Some brain damaged tolower() implementations appear to want to
take the locale into account, and this may require taking some
insanely aggressive lock on the locale and slowing down what should
be the most trivial of trivial calls for people who just want to
downcase ASCII.

Make our overflow checking look more like gcc and clang's, so that
we can substitute it out with the compiler instrinsics on platforms
that support it.  This means dropping the ability to pass `NULL` as
an out parameter.

As a result, the macros also get updated to reflect this as well.

Add some helper functions to check for overflow in a type-specific
manner.

Ensure that the given length to `p_read` is of ssize_t and ensure
that callers test the return as if it were an `ssize_t`.

Have the ALLOC_OVERFLOW testing macros also simply set_oom in the
case where a computation would overflow, so that callers don't
need to.