Copter: Loiter Turns should support circle radius, speed, yaw

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Some requests from msk7-ripe transferred over from Issue #707.

Some more about desired circle WPs behavior.
These WPs should have specific parameters
1 circle radius,
2 circle turns number (for loiter_turns) which should be non-integer value to allow circle leaving in any point,
OR circle time for loiter_time

3 circling angular speed (signed degrees/sec to include direction),
4 circle YAW mode (do not change yaw or direct yaw to center, may be tangent and outside yaw directions).
When AP sees circling WP in mission it should:
1 fly toward circle center at desired speed until reaching circle radius distance from it
2 start circling according to parameters
3 leave circle at desired turn/angle or desired time towards next WP

Start with issue #990 and clarify the requested loiter-turn parameters: radius, fractional turns or time, signed angular speed, and yaw mode. Done means a mission can approach the circle, execute the requested circling behavior, and leave toward the next waypoint; no source files or tests are identified in the issue.

Written by the indexing model from the issue text.