Fix "make checkdpadd" in usr.bin/iscsictl by removing -lfl dependency

Approved by: rpaulo (mentor)
MFC after: 1 week
Garbage collect libl dependency

The application links and runs without libl

Approved by: rpaulo (mentor)
Phabric: D673
X-MFC with: r270117
Submitted by: trociny
Return the spurious interrupt vector (IRQ7 or IRQ15) if the atpic cannot
find any unmasked pin with an interrupt asserted.

Reviewed by:	tychon
CR:		https://reviews.freebsd.org/D669
MFC after:	1 week