slist: add sys_slist_get() to fetch and remove the head

More straightforward than doing sys_slist_peek_head() followed by
sys_slist_remove().

Also add a version that does not check if the list is non-empty to be
used when the list is known bo be non-empty.

Change-Id: I8fd10e20e2c84c7d8972c9207f3d4917884808cb
Signed-off-by: Benjamin Walsh <benjamin.walsh@windriver.com>