Fix many clang errors in UIUC code

Fix many clang errors in UIUC code

- because uiuc_warning_error always called exit(-1), it can be marked
  no return, so Clang realises that when it’s used in the ‘else’
  side of a parsing test, uninitialised variables in the enclosing
  call site are safe.

(Requires Simgear update to define SG_NO_RETURN helper)