nginx compatability

[non7top]

Please add an option for a compatability mod with nginx. Two changes are neccesary:

1. Different header
   -#define AP_XSENDFILE_HEADER "X-SENDFILE"
   +#define AP_XSENDFILE_HEADER "X-Accel-Redirect"
2. Files are relative to docroot

• rv = ap_xsendfile_get_filepath(r, conf, file, &translated);

• /* make file relative */
• char *docroot = ap_document_root(r);

• apr_pool_t *mp;
• apr_pool_create(&mp, NULL);
• const char *cat_str = apr_pstrcat(mp, docroot, file, NULL);

• rv = ap_xsendfile_get_filepath(r, conf, cat_str, &translated);
• apr_pool_destroy(mp);

This patch may be ugly, but seems to be working fine. Would be great if this be controllable by the config directive.