Reshuffle ifdef HAVECACHE/DIAGNOSTIC blocks with the main purpose to make
code build without HAVECACHE/DIAGNOSTIC options, as well as some cleanup:
flags in pv_link is used for HAVECACHE only, declare/define it in the block.
flush_by_page declaration can omit ifdef when only conditional body is guarded.
segva is used only in DIAGNOSTIC/HAVECACHE in pmap_protect_mmu.
combine consecutive DIAGNOSTIC/HAVECACHE blocks were possible.

No functional changes intended.

(andvar)