fix #3712: strip trailing dot from searchdomain

fix #3712: strip trailing dot from searchdomain

having a trailing '.' in the search domain is perfectly legal syntax
(for domain names in general). postfix refuses to use a fqdn with
trailing dot as hostname[0].

The restriction might be due to section 2.3.5 (Domain Names) of
RFC5321 (a top-level domain is a single string without any dots) [1]

[0] src/util/valid_hostname.c in the postfix source
[1] https://datatracker.ietf.org/doc/html/rfc5321#section-2.3.5

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>