httpcaddyfile: Support repeated use of cert_issuer global option

This changes the signature of UnmarshalGlobalFunc but this is probably OK since it's only used by this repo as far as we know.

We need this change in order to "remember" the previous value in case a global option appears more than once, which is now a possibility with the cert_issuer option since Caddy now supports multiple issuers in the order defined by the user.

Bonus: the issuer subdirective of tls now supports one-liner for "acme" when all you need to set is the directory:

issuer acme <dir>

modules/caddytls/acmeissuer.go

@@ -233,7 +233,7 @@ func (iss *ACMEIssuer) GetACMEIssuer() *ACMEIssuer { return iss }
 
 // UnmarshalCaddyfile deserializes Caddyfile tokens into iss.
 //
-//     ... acme {
+//     ... acme [<directory_url>] {
 //         dir <directory_url>
 //         test_dir <test_directory_url>
 //         email <email>
@@ -250,9 +250,18 @@ func (iss *ACMEIssuer) GetACMEIssuer() *ACMEIssuer { return iss }
 //
 func (iss *ACMEIssuer) UnmarshalCaddyfile(d *caddyfile.Dispenser) error {
 	for d.Next() {
+		if d.NextArg() {
+			iss.CA = d.Val()
+			if d.NextArg() {
+				return d.ArgErr()
+			}
+		}
 		for nesting := d.Nesting(); d.NextBlock(nesting); {
 			switch d.Val() {
 			case "dir":
+				if iss.CA != "" {
+					return d.Errf("directory is already specified: %s", iss.CA)
+				}
 				if !d.AllArgs(&iss.CA) {
 					return d.ArgErr()
 				}