libc: Fix getentropy POSIX 2024 conformance issues

GETENTROPY_MAX should be defined in limits.h. EINVAL is the return
value for buflen > GETENTROPY_MAX.

PR: 282783
Reviewed by: markj, asomers, jhb
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D47689