Make KENV_MVALLEN tunable

[sjg]

Summary

When doing secure boot, loader wants to export loader.ve.hashed
the value of which typically exceeds KENV_MVALLEN.

Replace use of KENV_MVALLEN with tunable kenv_mvallen.

Add getenv_string_buffer() for the case where a stack buffer cannot be
created and use uma_zone_t kenv_zone for suitably sized buffers.

Obtained from: Abhishek Kulkarni <[e-mail]>
Sponsored by: Juniper Networks

[kevans]

This looks good to me.

I do wonder about the scenario (pre-existing): if some module were to kern_getenv() before the dynamic kenv was prepped and attempted to freeenv() after it was ready, we'd do the wrong thing. That's perhaps an unlikely scenario, though; the dynamic environment is established pretty early in boot, just after SI_SUB_KMEM).