bpftool: Exit from the stream wait signal handler with _exit()

bpftool prog tracelog -w ends from its SIGINT, SIGHUP and SIGTERM handler,
which calls exit(). exit() is not async-signal-safe: it runs atexit
handlers and flushes stdio streams, and the signal may land while the read
loop is inside fwrite() or fflush() on the stream being flushed. Whether
that deadlocks or flushes a half-updated buffer depends on the C library.

Call _exit() instead. The loop flushes every chunk as soon as it is
printed, so the stdio buffer is empty except while a chunk is being
written, and skipping the exit-time flush loses nothing.

While here, note in the manual page that opening a stream as a file
descriptor arrives in Linux 7.4, and restrict the -w/--wait description to
the stdout/stderr form of the tracelog command, since the trace pipe form
does not take the option.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Quentin Monnet <qmo@kernel.org>
Link: https://lore.kernel.org/bpf/d64c6402-175f-4ea6-be6d-1a544a95c97c@qmon.net
Link: https://lore.kernel.org/bpf/20260926154515.191689-1-memxor@gmail.com

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