MEDIUM: Add parsing of mailers section

As mailer and mailers structures and allow parsing of
a mailers section into those structures.

These structures will subsequently be freed as it is
not yet possible to use reference them in the configuration.

Signed-off-by: Simon Horman <horms@verge.net.au>
diff --git a/Makefile b/Makefile
index 4671759..4e3e166 100644
--- a/Makefile
+++ b/Makefile
@@ -664,7 +664,7 @@
        src/session.o src/hdr_idx.o src/ev_select.o src/signal.o \
        src/acl.o src/sample.o src/memory.o src/freq_ctr.o src/auth.o \
        src/compression.o src/payload.o src/hash.o src/pattern.o src/map.o \
-       src/namespace.o
+       src/namespace.o src/mailers.o
 
 EBTREE_OBJS = $(EBTREE_DIR)/ebtree.o \
               $(EBTREE_DIR)/eb32tree.o $(EBTREE_DIR)/eb64tree.o \