Make KENV_MVALLEN tunable

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.

Reviewed by: stevek, kevans
Obtained from: Abhishek Kulkarni <abkulkarni@juniper.net>
MFC after: 1 week
Sponsored by: Juniper Networks
Differential Revision: https://reviews.freebsd.org//D25259