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.
commit dbedca69c8030bb90eb38c833174e8bfbb65b4e4
fmtree: add a deprecation notice to the manpage MFC-after: 3 days
commit f10a4e8377edb5b8350ab1d44c415f84fe013f76
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.
commit b17ff8bfca4ab8ec67c567021e7b224624adab73
Regenerate src.conf(5) after FMTREE default change