net-mgmt/xymon-server: Fix vmstat parsing

The parsing of vmstat code was broken. The original code attempted to
detect FreeBSD 4.10's vmstat output vs FreeBSD 6, but probably never
worked correctly anyway.

FreeBSD 4.12's vmstat output:

 procs memory page disks faults cpu
 r b w avm fre flt re pi po fr sr ad0 md0 in sy cs us sy id
 0 0 0 10908 991180 68 1 1 0 55 0 0 0 241 201 12 0 1 99

FreeBSD 14.3 vmstat output:

procs memory page disks faults cpu
r b w avm fre flt re pi po fr sr nda0 nda1 in sy cs us sy id
1 2 0 7627545837568 4208828416 21155 13 15 1 14995 3542 0 0 1964 61975 39820 3 2 93

It's the same number of fields in the same order, so the vmstat data
was always being parsed into the struct for FreeBSD 4.11 which notably
skipped index 14 (sy) "cpu_syc" so it never had data for this and mapped
index 16 (us) to "cpu_sys" and index 17 (sy) to "cpu_usr" which are
obviously swapped.

With this patch I deleted the vmstat rrd files for FreeBSD hosts and the
data started to populate and graph correctly.