Moving FontforgeInternal

Having a directory inside the bin is generally frowned upon, and causes
issues with Homebrew due to our audit red-flagging that behaviour. This
(theoretically) changes that by moving that internal directory to
libexec/bin. This should work okay, theoretically. Let me know what you
all think.
Merge pull request #1838 from DomT4/internalpatch

Moving FontforgeInternal