Rename some existing variables/methods in the navcomm module for

Rename some existing variables/methods in the navcomm module for
clarity:

  nav_radial => nav_target_radial (same as selected, except for a LOC)
  nav_heading => nav_reciprocal_radial
  nav_magvar => nav_twist (it's not always the same as magvar)
  nav_heading_needle_deflection => nav_cdi_deflection
  nav_gs_needle_deflection => nav_gs_deflection

Added nav_radial back in, but now it shows the current radial from the
VOR, as one would expect.  This value also appears in the
/radios/nav[*]/radials/actual-deg property.