This is staged as three commits, but squashed here because none of them are substantial. I suspect I'll let the deprecation notice settle for a couple weeks, then axe it.
fmtree: add a deprecation notice to the manpage MFC-after: 3 days
build: turn off FMTREE by default to prepare for removal nmtree is derived from fmtree, and has been the default mtree(8) since 6adfbbbf161, a little over a year after its introduction. fmtree has not seen any substantial work since then, except for build fixes and runtime issues that were diagnosed in nmtree and backported because this was still in the tree. Turn it off by default.
Regenerate src.conf(5) after FMTREE default change