Require TUs to explicitly request to overwrite a pkgbase

AUR_PRIVILEGED allows people with privileged AUR accounts to evade the
block on non-fast-forward commits. While valid in this case, we should
not do so by default, since in at least one case a TU did this without
realizing there was an existing package.
( https://aur.archlinux.org/packages/rtmidi/

 )

Switch to using allow_overwrite to check for destructive actions.
Use .ssh/config "SendEnv" on the TU's side and and sshd_config
"AcceptEnv" in the AUR server to specifically request overwrite access.
TUs should use: `AUR_OVERWRITE=1 git push --force`

Signed-off-by: Eli Schwartz <eschwartz@archlinux.org>
Signed-off-by: Lukas Fleischer <lfleischer@archlinux.org>