Extend support for fifosignal/fifowait.

Now pressing F12 will signal the fifo in the the directory of the
current file instead of $PWD.  This way, it's easier to have multiple
fifos working together in a single Vim session.

Also, reference implementations of fifosignal and fifowait are now
embedded in vimfiles.
Catch E42 as another "no next/prev message" exception.

Also, on success echo an empty line to clear any previously written
message.
user/jszakmeister: setup a localvimrc whitelist