Static: supporting new "index" option.

This supports a new option "index" that configures a custom index
file name to be served when a directory is requested.  This
initial support only allows a single fixed string.  An example:

{
	"share": "/www/data/static/$uri",
	"index": "lookatthis.htm"
}

When <example.com/foo/bar/> is requested,
</www/data/static/foo/bar/lookatthis.html> is served.

Default is "index.html".

===

nxt_conf_validator.c:

Accept "index" as a member of "share", and make sure it's a string.

===

I tried this feature in my own computer, where I tried the
following:

- Setting "index" to "lookatthis.htm", and check that the correct
  file is being served (check both a different name and a
  different extension).
- Not setting "index", and check that <index.html> is being
  served.
- Settind "index" to an array of strings, and check that the
  configuration fails:

{
	"error": "Invalid configuration.",
	"detail": "The \"index\" value must be a string, but not an array."
}

src/nxt_http.h

@@ -218,6 +218,7 @@ typedef struct {
     nxt_conf_value_t                *location;
     nxt_conf_value_t                *proxy;
     nxt_conf_value_t                *share;
+    nxt_conf_value_t                *index;
     nxt_str_t                       chroot;
     nxt_conf_value_t                *follow_symlinks;
     nxt_conf_value_t                *traverse_mounts;