Avoid libprocstat warning messages on FreeBSD
Avoid libprocstat warning messages on FreeBSD

When trying to receive process state for a process that no longer exists,
libprocstat prints "capsicum-test: sysctl(kern.proc): No such process".
Avoid this output by checking with sysctl() first.
This is caused by a warn("sysctl(kern.proc)") call in procstat_getprocs()
in lib/libprocstat/libprocstat.c.

google/capsicum-test,google/capsicum-test,google/capsicum-test,google/capsicum-test