Set process vmem_size and stack_size on FreeBSD

Set process vmem_size and stack_size on FreeBSD

   Fixes:
   processes.c: In function 'ps_read':
   processes.c:328: warning: 'pse.vmem_size' is used uninitialized in this function
   processes.c:1300: note: 'pse.vmem_size' was declared here
   processes.c:330: warning: 'pse.stack_size' is used uninitialized in this function
   processes.c:1300: note: 'pse.stack_size' was declared here

Signed-off-by: Doug MacEachern <dougm@hyperic.com>
Signed-off-by: Florian Forster <octo@huhu.verplant.org>
Cherry-picked *again* because the initial commit to the collectd-4.5
branch was incorrect. collectd-4.6 is the branch this should be in.

Conflicts:

	src/processes.c