d/postfix.postinst: tolerate search domain with a leading dot

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.

[1] https://sourceware.org/git/?p=glibc.git;a=blob;f=resolv/res_query.c;h=ebbe5a6a4ed86abe3fccd4a134bfcf6f613c9bbb;hb=HEAD#l411

Closes: #991950