localedef: Const correctness for C23

[ivy]

Summary

On some platforms, e.g. Linux Clang 22.1.8 / glibc 2.43, strchr()
now implements the C23 behaviour where passing a const pointer to
strchr() also returns a const pointer. This breaks localedef during
the bootstrap build, since it assumes the return value is always
a mutable pointer.

This code can modify either a const (rodata) string, which happens
to be avoided by accident, or modify const global data. Change the
logic to instead create a local copy of the string and modify that
if needed. This introduces an additional allocation, but this is
not performance-sensitive code.

MFC after: 1 week
Sponsored by: The FreeBSD Foundation

[markj]

I think it'd be cleaner to drop the const qualifier from yesstr/nostr/yesexpr/noexpr and use strdup() to assign the empty string in this function. But this works too.

[ivy]

i was trying to avoid adding extra allocations for this group of patches, but i suppose this is not a performance-critical code path, so i'll switch it to use strdup if we think that's neater (i tend to agree, since __DECONST is at least a code smell).

[ivy]

use strdup() instead of __DECONST