privsep: Fix valgrind and hardened-malloc on Linux with SECCOMP

privsep: Fix valgrind and hardened-malloc on Linux with SECCOMP

Valgrind will still error by default at exit as the syscall to
unlink the pipe files is denied.
This can be avoided by compiling with -DVALGRIND.
The pipe files still won't be removed as dhcpcd has
already dropped to the non root user.
This is a Vagrind issue really.

hardened-malloc should now run as well as their documented syscalls
are now allowed by default.

Fixes #497.