api: comment, simplify and fix api socket read

api: comment, simplify and fix api socket read

The function vl_socket_read_ready did contain some comments already,
but as they stated, the logic has to be tricky to cover multiple cases.

Comment:
+ Add function-level comment
+ Add comments to describe some of local variables
+ Add many comments to describe internal state at particular lines.

Simplify:
+ Remov mbp_set as it is never needed.
+ Replace msg_len with msgbuf_len to save "+ sizeof (msgbuf_t)".

Improve:
+ Early exit on EAGAIN.

Fix:
+ "n" now only tracks input_buffer.
  Previously, it was entering the detection of additional messages
  even for unprocessed_input.
+ Set up msg_buffer (including appending to unprocessed_input)
  outside full-message-detection loop now,
  so it cannot be executed multiple times as before.

Type: fix
Ticket: VPP-1785

Change-Id: I256e34b435be06844458744a13ea37a0e86a96f9
Signed-off-by: Vratko Polak <vrpolak@cisco.com>