No sanity checks on FQDN(s)

[dnmvisser]

The scripts requires a space separated list of FQDNs, but there is no sanity check on them.
This can cause issues, for example when helpful software turns an FQDN into a URL. In that case the script outputs just a private key, and no CSR, and it also does not complain in any other way:

dick.visser@GA0267 ~$ genx509.sh https://foo.bar
Select signing type
1) CA-signed *
2) Self-signed
#? 
Select key type
1) ECC *
2) RSA
#? 
Select curve
1) prime256v1 *
2) secp384r1
#? 
-----BEGIN PRIVATE KEY-----
MIGHAgEAMBMGByqGSM49AgEGCCqGSM49AwEHBG0wawIBAQQgNfi8J1lJwYg4kInM
eVG6e5xEN4ZNkMoBwYatnaqKUMOhRANCAAR5MypGrbzW+FQ5Hx48qcsnhFB3f8S+
y5/z88gNztvyfPOY5TKVGgrQ0jVGg+lJRnfghxGAXPHTZciSsPjkll5Q
-----END PRIVATE KEY-----

The arguments need to be validates for correct FQDNs.