Add support for using external 3-way merge tool when handling @config items

[arrowd]

The introduced MERGETOOL config option expects a value in form "/path/to/program %b %l %r %o" where

• "%b" is the file containing the original config shipped by the previous version of the package
• "%l" is the file containing the current config (%b that is possibly altered by the user)
• "%r" is the new config that is going to be installed by the new package
• "%o" is the file that should be created by the too and contain the merge result
• "%n" is the name of file as listed in the pkg-plist with leading '/' stripped

If the merge tool return a non-zero code, pkg will attempt to the merging using a builtin algorithm.

[arrowd]

So, no need for rootfd in merge_with_external_tool() since all files it creates are in /tmp?