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