Commit an update to the pipe code that implements various pipe algorithms.

Commit an update to the pipe code that implements various pipe algorithms.
Note that the newer algorithms are either experimental or only exist for
testing purposes.  The default remains the same (sfbuf mode), which is
considered to be stable.  The code is just too useful not to commit it.

Add pmap_qenter2() for installing cpu-localized KVM mappings.

Add pmap_page_assertzero() which will be used in a later diagnostic commit.