Modify swapon(8) to invoke BIO_DELETE to trim swap devices, either if
'-E' appears on the swapon command line, or if "trimonce" appears as
an fstab option.

Discussed at: BSDCAN
Tested by: markj
Reviewed by: markj
Approved by: markj (mentor)
Differential Revision:https://reviews.freebsd.org/D20599