ddclient: T5612: Relax hostname validation for apex and wildcard entry

Some porvides (like 'namecheap') allow to use '@' or '*' as hostname
prefix for apex and wildcard records. This commit relaxes the hostname
validation to allow these prefixes.

(cherry picked from commit f0a8d0d8c622e67ef66c370747f13d4fdb6fd773)