Re: fix more casting away of qualifiers

Hi Peter,

I like this cleanup, especially removing the need to cast const char *
to char * just to call a function that only reads the input.

> The first two patches address the issue that unconstify cannot be used
> for global variables.

With GCC 10.2 and Clang 15, I checked that unconstify_constexpr works
in static initializers and that both unconstify_constexpr and
unconstify still reject casts to an incompatible pointer type.
V2 0001-0007 compiled with both compilers. I enabled OpenSSL and
GSSAPI in both builds to cover their API casts in 0004.

I also ran the same tests with both builds to check for behavior
changes: core regression, postgres_fdw, psql and PL/Python. All passed.

One small thing in 0005, in src/backend/utils/adt/float.c: the comment
above float8in_internal() still says:

> "num" could validly be declared "const char *", but that results in an
> unreasonable amount of extra casting both here and in callers,
> so we don't.

Could we remove those two lines now that num is const char *?

Otherwise, 0001-0007 look good to me.

Regards,
Rui