mail/dbmail: unbreak on FreeBSD15+, take maintainership (+)

On FreeBSD 15+ with recent ZFS in a box we always have libzdb.so and zdb.h installed
which conflicts with databases/zdb and breaks the build of dbmail

Set workaround to use <zdb/zdb.h> instead of plain <zdb.h>

PR: 291389
MFH: 2025Q4
Approved by: bofh (former maintainer)

(cherry picked from commit 25dc1d2cd656ece41ad917e0ce68b82969846242)