doas: enable timestamp by default and set pamdir

* `--with-timestamp` enables the usage of the `persist` setting in
`doas.conf`. It is possible some people might not want this, so the flag
`withTimestamp` was added to control this.
* `--pamdir` copies the PAM files to `$out/etc/pam.d`. This may or may
not have a use in the future, but it removes a some errors from the
build (when it tries to copy these files to /etc/pam.d).