req_ctx: Convert from hand-coded linked lists to linuxlist.h

This also removes the actual queuing code from req_ctx, and
leaves this to the caller.  Only tw out of the existing states actually
required the ordering requirement enforced by the qeue, and in the
future we will need to have per-endpoint queues anyway.  Both means it
is better to manage queues outside the req_ctx, and leve that as a pure
memory allocator.

firmware/Makefile

@@ -111,7 +111,7 @@ CFLAGS += #-Wpacked
 CFLAGS += -Wredundant-decls -Wnested-externs -Winline #-Wlong-long
 CFLAGS += -Wunreachable-code
 CFLAGS += -Wcast-align
-CFLAGS += -std=c11
+#CFLAGS += -std=c11
 CFLAGS += -Wmissing-noreturn
 #CFLAGS += -Wconversion
 CFLAGS += -Wno-unused-but-set-variable -Wno-unused-variable