REORG: trace: rename trace.c to calltrace.c and mention it's not thread-safe

The function call tracing code is a quite old and was never ported to
support threads. It's not even sure whether it still works well, but
at least its presence creates confusion for future work so let's rename
it to calltrace.c and add a comment about its lack of thread-safety.
diff --git a/Makefile b/Makefile
index 419d3ea..e19e549 100644
--- a/Makefile
+++ b/Makefile
@@ -785,7 +785,7 @@
               $(EBTREE_DIR)/ebimtree.o $(EBTREE_DIR)/ebistree.o
 
 ifneq ($(TRACE),)
-OBJS += src/trace.o
+OBJS += src/calltrace.o
 endif
 
 ifneq ($(EXTRA_OBJS),)
@@ -837,7 +837,7 @@
 # rebuild it every time
 .PHONY: src/version.c
 
-src/trace.o: src/trace.c $(DEP)
+src/calltrace.o: src/calltrace.c $(DEP)
 	$(cmd_CC) $(TRACE_COPTS) -c -o $@ $<
 
 src/haproxy.o:	src/haproxy.c $(DEP)