Renamed nxt_http_static_ctx_t field 'index' to 'share_idx'.
Renamed nxt_http_static_ctx_t field 'index' to 'share_idx'.

Having a configurable index filename will require adding an index
field to this structure.  The most natural name for that field is
'index', so the current index field should be renamed to allow for
that.  A sensible name is 'share_idx', since it's the index of the
shares array in 'nxt_http_static_conf_t'.

Instead of 'share_index' I opted for the shorter 'share_idx'.
Also, when 'index' allows an array of filenames in a following
commit, another similar variable 'index_idx' should be created,
and having a different prefix and suffix seems more readable than
for example 'index_index'.

nginx/unit,nginx/unit,nginx/unit,nginx/unit,nginx/unit,nginx/unit,nginx/unit,nginx/unit,nginx/unit,nginx/unit,nginx/unit