[Feature #20205]

As a path toward enabling frozen string literals by default in the future,
this commit introduce "chilled strings". From a user perspective chilled
strings pretend to be frozen, but on the first attempt to mutate them,
they lose their frozen status and emit a warning rather than to raise a
`FrozenError`.

Implementation wise, `rb_compile_option_struct.frozen_string_literal` is
no longer a boolean but a tri-state of `enabled/disabled/unset`.

When code is compiled with frozen string literals neither explictly enabled
or disabled, string literals are compiled with a new `putchilledstring`
instruction. This instruction is identical to `putstring` except it marks
the String with the `STR_CHILLED (FL_USER3)` and `FL_FREEZE` flags.

Chilled strings have the `FL_FREEZE` flag as to minimize the need to check
for chilled strings across the codebase, and to improve compatibility with
C extensions.

Notes:
  - `String#freeze`: clears the chilled flag.
  - `String#-@`: acts as if the string was mutable.
  - `String#+@`: acts as if the string was mutable.
  - `String#clone`: copies the chilled flag.

Co-authored-by: Jean Boussier <byroot@ruby-lang.org>

We've deleted opt_call_c_function instruction before.

This seems to be used nowhere today.

* Remove obsoleted opt_call_c_function insn

* Keep opt_call_c_function with DEFINE_INSN_IF

According to MSVC manual (*1), cl.exe can skip including a header file
when that:

- contains #pragma once, or
- starts with #ifndef, or
- starts with #if ! defined.

GCC has a similar trick (*2), but it acts more stricter (e. g. there
must be _no tokens_ outside of #ifndef...#endif).

Sun C lacked #pragma once for a looong time.  Oracle Developer Studio
12.5 finally implemented it, but we cannot assume such recent version.

This changeset modifies header files so that each of them include
strictly one #ifndef...#endif.  I believe this is the most portable way
to trigger compiler optimizations. [Bug #16770]

*1: https://docs.microsoft.com/en-us/cpp/preprocessor/once
*2: https://gcc.gnu.org/onlinedocs/cppinternals/Guard-Macros.html

for simplicity and consistency.

Now SUPPORT_JOKE needs to be prefixed with OPT_ to make the config
visible in `RubyVM::VmOptsH`, and the inconsistency was introduced.

As it has never been available for override in configure (no #ifndef
guard), it should be fine to rename the config.