asearch: Add context pointer to asearch comparison callback

asearch: Add context pointer to asearch comparison callback

asearch, like the standard library bsearch, takes a comparison callback.
Like bsearch() that callback doesn't include a user supplied context
pointer.  As well as being generally limiting, that makes the comparison
functions used by asearch gratuitously different from those used by the
asort module.

This patch alters this.  Note that this is an incompatible change to the
asearch interface.  I think this is worth it to correct the oversight, but
others might disagree.  At present the only user within ccan is ntdb, which
is corrected to match.

This means actually supplying a binary search implementation, rather than
relying on bsearch() from the standard library.  We follow the lead of the
asort module and steal^H^H^H^H^Hadapt the implementation from glibc.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>