* Disallow names that start with '.' in IsDomainName()

* Also update packDomain()

Add 'in the type bitmap' to make clear where in the RR the error occurs.
Also use 'NSEC(3)' - as this code is shared between NSEC and NSEC3, the
first error used NSECx.

Technically backwards incompatible, but checking strings in errors as
bad practice (although this lib lacks library types).

See #1373

Signed-off-by: Miek Gieben <miek@miek.nl>

This was broken by PR: #1322

Make it public as TsigGenerateWithProvider and update the docs a little.
And TsigVerifyWithProvider also - tweak those docs also a little.

Signed-off-by: Miek Gieben <miek@miek.nl>