Replace the old unix_seqpacket and unix_seqpacket_exercise tests, which
were a little broken and not automatable, with unix_seqpacket_test.
It's coverage is a superset of the old tests and it uses ATF.  It
includes test cases for bugs kern/185813 and kern/185812.

PR:		kern/185812
PR:		kern/185813
Sponsored by:	Spectra Logic
MFC after:	2 weeks