simplify upstream parsing methods via ngx_conf_parse api

DESCRIPTION
---
The following functions have been dropped, using `ngx_conf_parse` instead.

    static ngx_array_t *ngx_dyups_parse_content(ngx_pool_t *pool, ngx_buf_t *buf);
    static ngx_int_t ngx_dyups_conf_read_token(ngx_pool_t *pool, ngx_buf_t *body, ngx_array_t *args);
    static ngx_array_t *ngx_dyups_copy_args(ngx_pool_t *pool, ngx_array_t *arglist);
    static ngx_int_t ngx_http_dyups_check_commands(ngx_array_t *arglist);

The following functions have been added to help parse upstream configuration.

    /* parse upstream name, e.g. "upstream dyhost" */
    static char *ngx_dyups_parse_upstream_name(ngx_conf_t *cf, ngx_buf_t *buf, ngx_str_t *name);
    /* parse upstream servers, syntax is the same as upstream {} block without `{' and `}' */
    static char *ngx_dyups_parse_upstream(ngx_conf_t *cf, ngx_buf_t *buf);

The intermedia structure `arglist` is dropped, `ngx_conf_parse` parses configuration via `ngx_buf_t` structure directly.

TEST
---
The following cases, which is commented in t/dyups.t by myself, is not runned:

    like(mhttp_post('/upstream/dyhost', 'server 127.0.0.1:8088;check interval=3000 rise=2 fall=5 timeout=1000 type=http default_down=false;', 8081),
         qr/success/m, '2013-03-25 10:29:48');
    like(mhttp_get('/', 'dyhost', 8080), qr/8088/m, '2013-03-25 10:39:02');
    like(mhttp_delete('/upstream/dyhost', 8081), qr/success/m, '2013-03-25 10:39:28');
    
    like(mhttp_post('/upstream/dyhost', 'server 127.0.0.1:8088;check interval=1000 rise=2 fall=5 timeout=1000 type=http default_down=true;', 8081),
         qr/success/m, '2013-03-25 10:49:44');

Other 61 cases without healtch check module have passed.