Replace most `toStringz`/`toUTF16z`/`toUTFz` usages with `tempCString*` ones.

Only non-trivial cases left when a refactoring is required to use a temporary buffer.
Also add `nothrow`/`@nogc` attributes.
Waiting for `@nogc` attributes in druntime.

See D-Programming-Language/druntime#887.
Also 'core.sys.posix.sys.stat.lstat' isn't `@nogc`.