Problem: We have `P_(BOOL|NUM|STRING)` macros to represent an option's type, which is redundant because `OptValType` can already do that. The current implementation of option type flags is also too limited to allow adding multitype options in the future.
Solution: Remove `P_(BOOL|NUM|STRING)` and replace it with a new `type_flags` attribute in `vimoption_T`. Also do some groundwork for adding multitype options in the future.
Side-effects: Attempting to set an invalid keycode option (e.g. `set t_foo=123`) no longer gives an error.
closes#8274
The parent commit tries a different approach, but that fails on Apple
Clang version:
Apple LLVM version 10.0.0 (clang-1000.11.45.5)
Target: x86_64-apple-darwin17.7.0
which somehow compiles the check_c_source_compiles() check, but then
complains during later compilation that __fpclassify is not defined
(regardless of "#include <math.h>").