main/musl: backport patch to fix isatty()

upstream: https://git.musl-libc.org/cgit/musl/commit/src/unistd/isatty.c?id=c94a0c16f08894ce3be6dafb0fe80baa77a6ff2a

This fix aligns the implementation more closely with glibc, which fixes a somewhat critical issue in elogind that relies on how glibc handles the isatty() call on a hung TTY.