api: string type to convert to vector

api: string type to convert to vector

Previous use of strndup() required user to remember to call free().
Now return a vector pointing directly to the API message string.
Of course user must remember to copy the string out if lifetime
is longer than API message lifetime.

Change-Id: Ib5e2b3d52d258e1a42ea9ea9a9e04abbe360e2bf
Type: fix
Signed-off-by: Ole Troan <ot@cisco.com>