geli+trim support

[Dewayne Geraghty]

Thank-you RW for clarifying the discussion and looking at both sides of
the issue.  I agree that having TRIM available is, in my case, a
preferred risk and thanks also to Wojciech for raising this as I was
surprised to find that neither geli nor gshsec support BIO_DELETE.

But whether to zero or generate random data per
"/usr/src/sys/geom/eli/g_eli.c" (around line 282), I sense a sysctl
coming... :)