Print SSH and X.509 fingerprints in console banner without need to login (#2481)
Print SSH and X.509 fingerprints in console banner without need to login (#2481)

Example output:

```
$ /usr/local/etc/rc.initial.banner

*** test-fw.localdomain: OPNsense 18.1.10 (amd64/OpenSSL) ***

 WAN (vtnet0)    -> v4/DHCP4: 172.30.23.2/24

 SSH: 256 SHA256:fcMIAgT/vZR/TWP0j8AFROTNnudkU1tP9sRhbsIa8vM (ECDSA)
 SSH: 256 SHA256:lDenOc5wy2WU0e6sSz2hR9nEFnMqx5c3u1F/pHxgJlY (ED25519)
 SSH: 2048 SHA256:dsw9srlQHL0hPJlEdR9rL769N30BTZgXG9gXbdZGOkU (RSA)
 HTTPS X.509 cert: SHA256 Fingerprint=F0:E6:EB:31:E8:87:AF:52:16:4E:84:05:3B:6C:03:2C:C1:DF:5A:E7:36:F4:32:44:3B:B5:57:63:97:45:C3:77
```

The list of fingerprints is appended after the interface list because
the interface list might be pretty long and thus would move the
fingerprints out of the screen which we don’t want.

Previously (#2427) I suggested to extract the X.509 certificate from the
xml config but the difficult part for me who is not so familiar with the
implementation of OPNsense is to find the certificate which is actually
used by the local web server. I found that `/var/etc/cert.pem` is used
in the configuration of the local web server and assume that this is the
easier way to implement this in the expectation that the file name does
not change without being also changed in this script and that the file
exists. If it does not exist, OpenSSL would complain with a useful error
message.

This commit is one piece to make fully trusted bootstrapping easier.
Related to: https://github.com/opnsense/core/issues/2427
Tested on: OPNsense 18.1.10-amd64
opnsense/core,opnsense/core,opnsense/core,opnsense/core,opnsense/core,opnsense/core