Add a function, memstr, which can be used to convert a buffer of
null-separated strings to a single string. This can be used to print the
full arguments of a process using execsnoop (from the DTrace toolkit) or
with the following one-liner:

dtrace -n 'syscall::execve:return {trace(curpsinfo->pr_psargs);}'

Note that this relies on the process arguments being cached via the struct
proc, which means that it will not work for argvs longer than
kern.ps_arg_cache_limit. However, the following rather non-portable
script can be used to extract any argv at exec time:

fbt::kern_execve:entry
{
    printf("%s", memstr(args[1]->begin_argv, ' ',
        args[1]->begin_envv - args[1]->begin_argv));
}

The debug.dtrace.memstr_max sysctl limits the maximum argument size to
memstr(). Thanks to Brendan Gregg for helpful comments on freebsd-dtrace.

Tested by:	Fabian Keil (earlier version)
MFC after:	2 weeks
Add bus space barriers to the AR71xx SPI code.

This is required for correct, stable operation on the MIPS74k SoCs
that are dual-issue, superscalar pipelines.

Tested:

* AR9344 SoC (MIPS74k)
* AR9331 SoC (MIPS24k)