ez-ipupdate: Add Namecheap dynamic DNS support

This does not include any web interface changes.

Namecheap login works a little differently than other dynamic DNS
services. Namecheap only requires the subdomain, domain and a
randomly generated password. These are roughly equivelent to other
services host, username and password, respectively.

Examples:
Update 'www.example.com'
ez-ipupdate -S namecheap -h www -u example.com:e747d77054a844409c486973cb
Update 'example.com'
ez-ipupdate -S namecheap -h @ -u example.com:e747d77054a844409c486973cb

It is unclear if Namecheap supports updating wildcard addresses with
this method, at this time. Support for wildcards will be added if they
are supported by Namecheap.
Web interface: Add Namecheap dynamic DNS support

The subdomain (or @ for the root domain) goes in the Host Name field.
The domain goes in User Name/Email Address field.
The passkey provided by Namecheap goes in the Password/DDNS Key field.

Wildcard is currently unavailable.
DHCPv6: Request a /56 IA_PD prefix
Make sla-len config optional
Make sla-id config optional
DHCPv6: Don't set sla-len