Add with-sessiond-bin configure option

I also cleaned up the little check_sessiond function. The check using
access(2) was only done for the command line option, but I think it is a
good idea to do it wherever the path comes from.

Closes #441

Signed-off-by: Simon Marchi <simon.marchi@polymtl.ca>
Signed-off-by: David Goulet <dgoulet@efficios.com>