libc, libthr: use AT_USRSTACK{BASE,LIM} instead of sysctl("kern.usrstack") and get_rlimit(RLIMIT_STACK)
Reviewed by: brooks, imp (previous version)
Discussed with: markj
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
Differential revision: https://reviews.freebsd.org/D36540