cdesf: Support _POSIX_C_SOURCE 202405 for POSIX 2024

Allow applications to request a strict POSIX 2024 by setting
_POSIX_C_SOURCE to 202405 or higher. The rest of the system needs tweaks
to be compliant, so this is a work in progress.

Sponsored by: Netflix
Reviewed by: brooks
Differential Revision: https://reviews.freebsd.org/D47575