Some problems were introduced during the libxo-ification of vmstat, fix them

stop vmstat -i segfaulting
remove duplicate header from vmstat -i
do not pad the name of the interupt in encoded outputs
fix stray % and missing } in the header for vmstat -i
add outer container to vmstat -i
add missing xo_flush in vmstat -i (when run with an interval or delay)
add outer container to vmstat -m
do not pad the name of malloc areans
add outer container to vmstat -z
do not pad the name of memory zones

Reviewed by: rodrigc
Approved by: bapt (mentor)
Sponsored by: ScaleEngine Inc.
Differential Revision: https://reviews.freebsd.org/D4263