configure now probes for <linux/openat2.h> + SYS_openat2 and defines
HAVE_OPENAT2 only when both are present; syscall.c gates the openat2 include
and the openat2(RESOLVE_BENEATH) tier on HAVE_OPENAT2, so the build no longer
fails on kernels/headers that lack the openat2 header (3.4.3 included it
unconditionally on Linux). android.c probes openat2 usability behind a SIGSYS
handler so the Android/Termux seccomp sandbox falls back to the portable
resolver instead of killing the process.
Backport combining c73e0063, 83a24c21, the syscall.c guards from 1d5b5ab8, and
4634b0ad; the --disable-openat2/gcov coverage knobs and test changes are omitted.
Thanks to @mmayer (#924), @fda77 (#905), @darkshram (#900) and @ketas (#904) for the reports.
