trim(8): emit more user-friendly error message in verbose mode.

If underlying driver provides no TRIM/UNMAP support and operation fails
due to this reason, state it clearly in verbose mode (default)
instead of writing standard message that may be too cryptic for a user:

trim: ioctl(DIOCGDELETE) failed: nda0: Operation not supported

Now it would write:

trim: nda0: TRIM/UNMAP not supported by driver

But still use previous format including errno value for quiet mode.

Small candelete() function borrowed from diskinfo(8) code.
This function was committed by Alan Somers <asomers@FreeBSD.org>,
so give him some credit.

Reported by: chuck