build-sys: Fix pidfd_open checking

The previous build setup would check for pidfd_open using
AC_CHECK_FUNC and would be incorrectly reported as true.

Also, if pidfd_open() and __NR_pidfd_open were not present,
pidwait would silently not be built.

So, changes:
 compile a small programin using pidfd_open to test it properly
 conditionally try to find NR_pidfd_open if the function fails
 complain if neither are present
 have --disable-pidwait configure option so you are explicit in
 not wanting and knowing you wont get pidwait

References:
 #352
 commit d9c3e367
 commit 17f94796

Signed-off-by: Craig Small <csmall@dropbear.xyz>