Introduce "enum meshms_status" to replace int -1,0,1 for return values
from MeshMS internal functions
Add test cases to check that status 2 is returned when attempting to
send a message from an identity that is not known (unlocked)
Include local (my) and remote (their) SIDs in JSON responses for
conversationlist.json and messagelist.json, so that the same JSON
structures can be used in future for non-SID-specific queries.
Refactor MeshMS message iterator, rename "sender" and "recipient" fields
to "my" and "their", for consistency with the rest of the MeshMS source
code, and because "sender" and "recipient" are properties that apply to
a single message or single ply, not to a message thread (conversation).
