Draft: ppp-udeb.postinst: Rework hostname code

Refine echo replacement with printf (fixes commit c1c728fa)

• Add missing trailing new line in format string
• Move variable substitution out of format string
• replace more echo with printf

Rework valid_hostname()

• Replace non-portable 'echo' with 'printf'

• Fix regex to not allow backslash

• Do not allow hyphens or dots at the beginning or the end of the hostname

• Do not allow hyphens at the beginning or the end of a label

• Add new function valid_domain() and reject invalid domain.

• Change valid_hostname() to allow only short hostname (no dots).