sendmail: use host rather than NIS domain name for HELO

sendmail: use host rather than NIS domain name for HELO

According to RFC 5321 the argument to HELO "contains the fully-qualified
domain name of the SMTP client" or its IP address if no FQDN is available.
BusyBox sendmail uses the NIS domain name instead which, in many cases,
is likely to be the default "(none)". [vda: yes, I checked my machine
and its uts.domainname was indeed "(none)"]

Using the host name is more likely to satisfy the intent of the RFC while
allowing the otherwise unused safe_getdomainname function to be removed.

Signed-off-by: Ron Yorston <rmy@tigress.co.uk>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>