Enable the ability to load multiple versions of the same TCP stack

[jtl]

Summary

This patch enables the ability to load multiple versions of the same TCP stack.

It lets us write simple Makefiles like this:

#
# $FreeBSD$
#

STACKNAME=      bbr_17q22
KMOD=   tcp_${STACKNAME}
SRCS=   bbr.c sack_filter.c

#
# Enable full debugging
#
#CFLAGS += -g

CFLAGS+=        -DMODNAME=${KMOD}
CFLAGS+=        -DSTACKNAME=${STACKNAME}
PREFIX_SYMS=    ${KMOD}_

.include <bsd.kmod.mk>

In the code, we can register non-conflicting module names like this:

MODULE_VERSION(MODNAME, 1);
DECLARE_MODULE(MODNAME, tcp_bbr, SI_SUB_PROTO_DOMAIN, SI_ORDER_ANY);

We can enable multiple stack names like this:

static const char *my_stack_names[] = {
        __XSTRING(STACKNAME),
#ifdef STACKALIAS
        __XSTRING(STACKALIAS),
#endif
};
...
                num_stacks = nitems(my_stack_names);
                err = register_tcp_functions_as_names(&__tcp_mystack, M_WAITOK,
                    my_stack_names, &num_stacks);

To do this, it:

• Supports symbol mangling to avoid overlapping symbols between the multiple modules.
• Lets us use a macro as the module name in the DECLARE_MACRO() and MACRO_VERSION() macros.
• Lets us register stack aliases (e.g. both a generic name ["default"] and version-specific name ["default_v1"]).

We can then create a new version of the module just by changing the STACKNAME variable.

Test Plan

This is in use in Netflix.

[sjg]

makefile changes look ok, thought the number of ${OBJCOPY} invocations might be getting out of hand.
Since they all work the same it would be possible to accumulate the output of all the awk commands in one file, and then feed that to xargs -J% ${OBJCOPY} % ${.TARGET}

[sjg]

never mind, objcopy -J% is going to be invoked once per line anyway

[sjg]

so makefiles ok