slist: add sys_slist_append_list and sys_slist_merge_slist()

Allow appending a list to another list. The list being appended can be
either a slist or the head and tail of a singly-linked list with the
same node format as the slist.

Change-Id: I14410d2b793e1d9f893ff4e7ce097bee4a93a4be
Signed-off-by: Benjamin Walsh <benjamin.walsh@windriver.com>