It is not necessary to traverse *busy and link the *out when *out is NULL.

On Linux, the rename syscall can be slow due to a global file system lock,
acquired for the entire rename operation, unless both old and new files are
in the same directory.  To address this temporary files are now created
in the same directory as the expected resulting cache file when using the
"use_temp_path=off" parameter.

This change mostly reverts 99639bfdfa2a and 3281de8142f5, restoring the
behaviour as of a9138c35120d (with minor changes).

Files are considered the same if the path used by nginx during parsing matches.

With this change it is now possible to load modules compiled without
the "--with-http_ssl_module" configure option into nginx binary compiled
with it, and vice versa (if a module doesn't use ssl-specific functions),
assuming both use the "--with-compat" option.

With this change it is now possible to load modules compiled without
the "--with-file-aio" configure option into nginx binary compiled with it,
and vice versa, assuming both use the "--with-compat" option.

With this change it is now possible to load modules compiled without
the "--with-threads" configure option into nginx binary compiled with it,
and vice versa (if a module does not use thread-specific functions),
assuming both use the "--with-compat" option.

Linux can return AF_UNIX sockaddrs with partially filled sun_path,
resulting in spurious comparison failures and failed binary upgrades.
Added proper checking of the lengths provided.

Reported by Jan Seda,
http://mailman.nginx.org/pipermail/nginx-devel/2016-September/008832.html.

Previously, user access bits were always set to "rw" unconditionally,
even with "user:r" explicitly specified.  With this change we only add
default user access bits (0600) if they weren't set explicitly.