MFC loader fixes

r361710: stand/uboot: fix setting of gateip.s_addr
Missplaced paren.

r361933: loader: install allow for more complete device spec in url

Rework to simplify and impose sane url syntax.
That is we allow for file://[devname[:fstype]]/package

r362127: verify_pcr_export: bump kenv_mvallen if needed
r362231: 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.

r364443: veloader: insist on verifying .4th .lua etc

When files are read from .rc or .4th, verify_file is asked to
guess the severity (VE_TRY,VE_WANT,VE_MUST)

Reviewed by: imp, stevek, kevans