libutil: eliminate one syscall from kinfo_getproc

Previously we invoked the sysctl with a NULL buffer to query the size,
allocated a buffer, then invoked it again to fetch the data.

As we only handle the case where the sysctl provides data of the
expected size we can just allocate a correctly-sized buffer to begin
with.

Reported by: Thomas Hurst via Twitter
Reviewed by: kevans
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D35140