net-mgmt/arpwatch: Update to 3.2

Brad Davis points out that sendmail won't exist if the system was
built with WITHOUT_MAIL which causes configure to fail; use the new
--with-sendmail configure option to hardwire sendmail to
/usr/sbin/sendmail.

Changes since 3.1:

• Change update-ethercodes to not clobber ethercodes.dat on failure.
• Add a configure option to specify the path to sendmail.

PR: 260420
Reported by: Brad Davis