rgw: fix setting of NULL to string

Fixes: #3777
s->env->get() returns char * and not string and can return NULL.
Also, remove some old unused code.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
Reviewed-by: Greg Farnum <greg@inktank.com>
(cherry picked from commit 9019fbbe8f84f530b6a8700dfe99dfeb03e0ed3d)
rgw_rest: Make fallback uri configurable.

Some HTTP servers, notabily lighttp, do not set SCRIPT_URI, make the fallback
string configurable.

Signed-off-by: caleb miles <caleb.miles@inktank.com>
Reviewed-by: Yehuda Sadeh <yehuda@inktank.com>
(cherry picked from commit b3a2e7e955547a863d29566aab62bcc480e27a65)