MFC r336152-r336154, r336157

r336152:
subr_hints: Use goto/label instead of series of conditionals

r336153:
subr_hints: Convert some bool-like ints to bools

r336154:
subr_hints: Skip static_env and static_hints if they don't contain hints

This is possible because, well, they're static. Both the dynamic environment
and the MD-environment (generally loader(8) environment) can potentially
have room for new variables to be set, and thus do not receive this
treatment.

r336157:
kern_environment: bool'itize dynamic_kenv; fix small style(9) nit