setusercontext: only call setrtable(2) if "rtable" set explicitly.

setusercontext: only call setrtable(2) if "rtable" set explicitly.
When LOGIN_SETRTABLE is set in flags, only call setrtable(2) if
there is an "rtable" entry in login.conf.  Previously, a default
value of 0 was used if the capability was missing.  That had the
effect of resetting the rtable for every program that uses
setusercontext(3), even if a different rtable was chosen by, e.g.,
"route -T exec command".  OK deraadt@ matthieu@