Relax the rule against declaring variables in nested scopes and for
initializations.

Relax some overly perscriptive rules against declarations: they may be at the
start of any blocks, even if things aren't super complicated. Allow more
initializations (those that call simple functions, like accessor functions for
newbus are fine). Allow the common idiom of declaring the loop variable in a for
loop.

This tries to codify what common exceptions are today, as well as give
some guidance on when it's best to do these things.

Reviewed by: tsoome, kp, markm, allanjude, jiles, cem, rpokala
(earlier versions: seanc, melifaro, bapt, pjd, bz, pstef, arichards,

	 jhibits, vangyzen, jmallet, ian, glebius, jhb, dab, adrian,
	 sef, gnn)

Differential Revision: https://reviews.freebsd.org/D25312