Validate hostnames given in `NJS_ACME_SERVER_NAMES`

[zsteinkamp]

Currently, we will split the value of NJS_ACME_SERVER_NAMES on space/comma and blindly use the result to request certs.

Since the HTTP-01 challenge type only supports full hostnames and not wildcard names, we should:

• document clearly that wildcards are not allowed
• explicitly test the values we get from that variable for validity as hostname characters (excluding *)
• display a clear and helpful error message in logs and in an early response to the /acme/auto endpoint if invalid values are detected.

[ivanitskiy]

we also need to consider checking the following:

• when the certificate is issued persisted on OS FS. nginx admin can update the config and append a new hostname (nginx var or env var NJS_ACME_SERVER_NAMES) we need to detect this and issue a new cert(s)

[zsteinkamp]

I'll open a new issue for that. 👍