add an option for ddb ps command to print process arguments

We use ps to collect the information of all processes in textdump. But
it doesn't contain process arguments which however sometimes are very
useful for debugging. The new 'a' modifier adds that capability.

While here, remove 'm' modifier from ddb.4. It was in the manual page
from its very first revision, but I could not find any evidence of the
code ever supporting it.

Submitted by: Terry Hu <thu@panzura.com>
Reviewed by: kib
MFC after: 1 week
Sponsored by: Panzura
Differential Revision: https://reviews.freebsd.org/D16603