I found lots of places where we assume error will be set when calling

I found lots of places where we assume error will be set when calling
one of our virtualised functions, such as db_open(), but error is only
set when a system call fails, and it is not uncommon for us to fail a
function internally without ever making a system call. That led to us
passing back success when a function had in fact failed.

I found two places where we relied on map_nt_error_from_unix()
returning success when errno==0, but lots and lots of places where we
relied on the reverse, so I fixed those two places.

map_nt_error_from_unix() will now always return an error, returning
NT_STATUS_UNSUCCESSFUL if errno is 0
(cherry picked from commit 69d40ca4c1af925d4b0e59ddc69ef8c26e6501d1)
(This used to be commit 834684a524a24bb4eb46b4af583d39947dc87d95)