MFC r351078, r351085, r351088: mostly a nop (two commits + revert of two)

This commit is mostly a nop, but ends up renumbering #4 clause to #3 in one
copy of quad.h... this is OK; stand/ situation in stable/11 is pretty murky
and the commit that renumbered the clause got lost somewhere. quad.h will be
disappearing in a not-so-distant future MFC.

r351078:
stand: kick out quad.h

Use quad.h from libc instead for the time being. This reduces the number of
nearly-identical-quad.h we have in tree to two with only minor changes.

Prototypes for some *sh*di3 have been added to match the copy in libkern.
The differences between the two are likely few enough that they can perhaps
be merged with little additional effort to bring us down to 1.

r351085:
libc quad.h: one last _STANDALONE correction

r351088:
Revert r351078, r351085: stand/quad.h eviction

It did not go well; further examination is required...