[9] RFR: 8007706: X.509 cert extension SAN should support "_" in dNSName

[Jason Uh]

Hi Max,

Could you please review this fix?

http://cr.openjdk.java.net/~juh/8007706/webrev.00/

With the fix, the rules will be:
1. A DNSName must begin with a letter or a digit
2. After the first character, valid characters in DNSName components are 
letters, digits, hyphens, and underscores

A couple minor formatting changes included.

Thanks,
Jason