chasquid: Do a Chdir into the configuration directory

In some cases, it's be useful to have references to directories relative
to the configuration itself.

So this patch makes chasquid do a Chdir into it, so we can assume it in
the rest of the code and config.

test/util/test-mda

@@ -2,13 +2,13 @@
 
 set -e
 
-mkdir -p .mail
+mkdir -p ${MDA_DIR}
 
 # TODO: use flock to lock the file, to prevent atomic writes.
-echo "From ${1}" >> .mail/.tmp-${1}
-cat >> .mail/.tmp-${1}
+echo "From ${1}" >> ${MDA_DIR}/.tmp-${1}
+cat >> ${MDA_DIR}/.tmp-${1}
 X=$?
-if [ -e .mail/.tmp-${1} ]; then
-	mv .mail/.tmp-${1} .mail/${1}
+if [ -e ${MDA_DIR}/.tmp-${1} ]; then
+	mv ${MDA_DIR}/.tmp-${1} ${MDA_DIR}/${1}
 fi
 exit $X