i#2224: handle win10's change in partial memory writes

On win10, NtWriteVirtualMemory no longer returns the number of bytes written and instead returns -1! Thus, if the caller of safe_write_ex() cares about that, we fall back to a try-except version rather than using the system call.

Clarifies the dr_safe_read and dr_safe_write functions: that they fail if the bytes read or written is partial. Updates all callers who want to fail on a partial read or write to pass NULL instead of taking time to check the size read/written.

Fixes #2224 (closed)