libbb: use _exit, not exit, in bb_daemonize_or_rexec()

By the time we reach exit in parent, child already exited or execed.
We should not re-run libc cleanup code.

While at it, introduce bb_daemon_helper() and add a few comments.

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

include/libbb.h

@@ -1169,6 +1169,7 @@ enum {
 #endif
 void bb_daemonize_or_rexec(int flags, char **argv) FAST_FUNC;
 void bb_sanitize_stdio(void) FAST_FUNC;
+#define bb_daemon_helper(arg) bb_daemonize_or_rexec((arg) | DAEMON_ONLY_SANITIZE, NULL)
 /* Clear dangerous stuff, set PATH. Return 1 if was run by different user. */
 int sanitize_env_if_suid(void) FAST_FUNC;