procstat auxv: widen COMM column to MAXCOMLEN

For reasons unknown, procstat subcommands typically display the command
in a 16+overflow column format. However, the command may be up to
MAXCOMLEN (19) characters long causing the column to spill into the next
one. Since there's plenty of room in the auxv case, bump the column
width up to 19 to avoid this issue. While this is a format change 1)
users who want to parse the data should use libxo output and 2) this
makes it possible to parse reliably with cut.

Reviewed by: markj
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D42202