On error, sbuf_bcat() returns -1.  Some callers returned this -1 to
the upper layers, which interpret it as errno value, which happens to
be ERESTART.  The result was spurious restarts of the sysctls in loop,
e.g. kern.proc.proc, instead of returning ENOMEM to caller.

Convert -1 from sbuf_bcat() to ENOMEM, when returning to the callers
expecting errno.

In collaboration with:	pho
Sponsored by:	The FreeBSD Foundation (kib)
MFC after:	1 week