frontend: fix -R trace

trace is a seperate runcore, but misses a _init registration
function, we rather go through the slow runcore and check
then the trace flag.
Fixes the t/run/options.t breakage of the previous commit

src/embed/api.c

@@ -1,5 +1,5 @@
 /*
-Copyright (C) 2010-2012, Parrot Foundation.
+Copyright (C) 2010-2014, Parrot Foundation.
 
 =head1 NAME
 
@@ -187,6 +187,8 @@ Parrot_api_set_runcore(Parrot_PMC interp_pmc, ARGIN(const char * corename),
     else {
         if (STREQ(corename, "slow") || STREQ(corename, "bounds"))
             Parrot_runcore_switch(interp, Parrot_str_new_constant(interp, "slow"));
+        else if (STREQ(corename, "trace"))
+            Parrot_runcore_switch(interp, Parrot_str_new_constant(interp, "slow"));
         else if (STREQ(corename, "fast")
                  || STREQ(corename, "function")
                  || STREQ(corename, "cgp")
@@ -202,8 +204,6 @@ Parrot_api_set_runcore(Parrot_PMC interp_pmc, ARGIN(const char * corename),
         else if (STREQ(corename, "exec"))
             Parrot_runcore_switch(interp, Parrot_str_new_constant(interp, corename));
 #endif
-        else if (STREQ(corename, "trace"))
-            Parrot_runcore_switch(interp, Parrot_str_new_constant(interp, corename));
         else if (STREQ(corename, "profiling"))
             Parrot_runcore_switch(interp, Parrot_str_new_constant(interp, corename));
         else if (STREQ(corename, "gc_debug"))

src/ops/set.ops

@@ -178,6 +178,7 @@ Assign a new value $2, to PMC $1. Same as the equivalent B<set> opcodes.
 =item B<assign>(invar PMC, invar PMC)
 
 Assign a new value $2, to PMC $1 by copying the value.
+Usually calls the set_pmc vtable method.
 
 =item B<assign>(out STR, in STR)