FAST_DEPEND/META_MODE: Fix several issues with SRCS containing '/'.

• For FAST_DEPEND, properly apply the -M flags when compiling by enduring that the condition also has the s,/,_,g filter applied to it first.
• For FAST_DEPEND, properly read from the filtered filename.
• For META_MODE, it needs to read from a full-pathed and s,/,_,g replaced filename based on bmake's meta_name() function which names the .meta file for SRCS with '/' in them.

This support has not been extended to the kernel build yet but may be in the
future.

MFC after: 2 weeks
Reported by: dim
Sponsored by: EMC / Isilon Storage Division