ovpnmain.cgi: Fixes Bug#13137 - Existing n2n client connection created with openssl...

ovpnmain.cgi: Fixes Bug#13137 - Existing n2n client connection created with openssl-1.1.1x fails to start with openssl-3.x

- With a n2n connection .p12 certificate created wityh openssl-1.1.1x the line
   providers legacy default is required in the n2nconf file to enable it to start.
- Any openssl-3.x attempt to open a .p12 file created with openssl-1.1.1x will result in
   a failure and an error message. All the openssl commands dealing with pkcs12 (.p12)
   files need to have the -legacy option added to them.

Fixes: Bug#13137
Tested-by: Adolf Belka <adolf.belka@ipfire.org>
Signed-off-by: Adolf Belka <adolf.belka@ipfire.org>
Reviewed-by: Michael Tremer <michael.tremer@ipfire.org>