Collapse mc_pos_control and mc_pos_control_multiplatform into one

[thomasgubler]

equivalent to #1809

[thomasgubler]

Here is a quick link for the history of mc_pos_control: https://github.com/PX4/Firmware/commits/master/src/modules/mc_pos_control

To be ported:

• mc_pos_control: Protect against NaN and Inf setpoints #1703 (lacking merge commit)
  • I suggest to add an extra callback for the pos_setpoint_triplet instead of the default copy-callback. Then the check introduced in this PR can be added there
  • Ported in port #1703 to multiplatform #1821
• mc attitude setpoint generation #1741 (lacking merge commit) Ported in Multiplatform 1741 and 1801 #1823
• log velocity - and acceleration/thrust setpoint #1752 ported in port #1752 to multiplatform #1822
• mc_pos_control: Fix autonomous landing without GPS #1801 (lacking merge commit) Ported in Multiplatform 1741 and 1801 #1823