processes plugin: Limit the buffer size used for command line matching.

ARG_MAX is quite big on many systems, for example >100 kByte on
GNU/Linux. This is a problem for systems with tight memory constraints,
for example embedded devices.

This patch uses at most 4 kByte for this, which out to be enough for the
vast majority of users. Users with specific requirements can compile
with "CMDLINE_BUFFER_SIZE=${LOTS}" in their CPPFLAGS to override this
default.

Fixes: #652
Merge remote-tracking branch 'github/master'