fix #2504: do not overwrite existing selector key

fix #2504: do not overwrite existing selector key

This patch changes the behavior of DKIM selector creation. Instead of blindly
overwriting an already present file, add a force parameter to overwrite it (and
behave like the current code).

Overwriting an existing selector can potentially be quite destructive (e.g.
a setup where the admin has already posted the DNS-record for one selector to
many domains, then wants to quickly experiment with a larger keysize, and tries
to go back to the existing behavior).

The new behavior without force set to true, when a private key for the selector
already exists is to die if the file is either not a private RSA key, or has
the wrong size, or else just set the selector in pmg.conf

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>