d/postfix.postinst: tolerate search domain with a leading dot
requested to merge paride/postfix-dev:debbug991950-accept-search-doms-w-leading-dot into debian/master
Search domain with a leading dot cause postfix.postinst to fail because it constructs a 'myhostname' with a duplicate dot (see #991950).
The glibc resolver tolerates such domains and strips the leading dot from the search domain [1]. This change makes postfix.postinst do the same.
Closes: #991950