util: git__memzero() tweaks

On Linux: fix a warning message related to the volatile qualifier (cast)
On Windows: use SecureZeroMemory()

On both, inline the call, so that no entry point can lead back to this "secure" memory zeroing.
cred: deploy git__memzero to clear memory holding a password
Merge pull request #1654 from yorah/memzero

Memzero stuffs