This includes non-absolute pathnames, multiple slashes and trailing
slashes.  In collaboration with Valentin Bartenev.

Notably this fixes NGX_INVALID_FILE/NGX_FILE_ERROR mess, and adds
logging of close() errors.  In collaboration with Valentin Bartenev.

To completely disable symlinks (disable_symlinks on)
we use openat(O_NOFOLLOW) for each path component
to avoid races.

To allow symlinks with the same owner (disable_symlinks if_not_owner),
use openat() (followed by fstat()) and fstatat(AT_SYMLINK_NOFOLLOW),
and then compare uids between fstat() and fstatat().

As there is a race between openat() and fstatat() we don't
know if openat() in fact opened symlink or not.  Therefore,
we have to compare uids even if fstatat() reports the opened
component isn't a symlink (as we don't know whether it was
symlink during openat() or not).

Default value is off, i.e. symlinks are allowed.

No functional changes.

Nuke NGX_PARSE_LARGE_TIME, it's not used since 0.6.30.  The only error
ngx_parse_time() can currently return is NGX_ERROR, check it explicitly
and make sure to cast it to appropriate type (either time_t or ngx_msec_t)
to avoid signedness warnings on platforms with unsigned time_t (notably QNX).

Now redirects to named locations are counted against normal uri changes
limit, and post_action respects this limit as well.  As a result at least
the following (bad) configurations no longer trigger infinite cycles:

1. Post action which recursively triggers post action:

    location / {
        post_action /index.html;
    }

2. Post action pointing to nonexistent named location:

    location / {
        post_action @nonexistent;
    }

3. Recursive error page for 500 (Internal Server Error) pointing to
   a nonexistent named location:

    location / {
        recursive_error_pages on;
        error_page 500 @nonexistent;
        return 500;
    }