Disallow underscores in site URLs/names

[theo-o]

From RFC 1738

Fully qualified domain names take the form as described
in Section 3.5 of RFC 1034 [13] and Section 2.1 of RFC 1123
[5]: a sequence of domain labels separated by ".", each domain
label starting and ending with an alphanumerical character and
possibly also containing "-" characters.

By the RFC, _ are disallowed in domain labels, hence Director should not support _ in site URLs.