WITH_META_MODE: Lessen the filemon(4) requirement scope.

- Move the sys.mk filemon requirement to bsd.init.mk as a warning.
  This is intended only to show when building directly in a subdirectory
  without filemon loaded.
- Move the error into Makefile and only apply it when building
  from the META_TGT_WHITELIST target list.

-DNO_FILEMON can be used to suppress both the warning and the error but
makes WITH_META_MODE less useful.  It will only compare build commands
in this mode rather than track all dependencies.

This fixes installing from a jail which doesn't need filemon in this
phase [1].

Reported by:	Nikolai Lifanov <lifanov@mail.lifanov.com> [1]
Approved by:	re (implicit)
Sponsored by:	EMC / Isilon Storage Division

share/mk/bsd.init.mk

@@ -28,4 +28,14 @@ _SKIP_BUILD = not building at level 0
 .warning ${_SKIP_BUILD}
 .endif
 
+.if ${MK_META_MODE} == "yes"
+.if !exists(/dev/filemon) && \
+    ${UPDATE_DEPENDFILE:Uyes:tl} != "no" && !defined(NO_FILEMON) && \
+    !make(showconfig)
+.warning The filemon module (/dev/filemon) is not loaded.
+.warning META_MODE is less useful for incremental builds without filemon.
+.warning 'kldload filemon' or pass -DNO_FILEMON to suppress this warning.
+.endif
+.endif	# ${MK_META_MODE} == "yes"
+
 .endif	# !target(__<bsd.init.mk>__)