bitcoind: formula rewrite

Makes building without a gui the default, as arguably the Cask or
Bitcoin themselves are better distributing the core GUI. Having a
formula that by default is CLI based, with only a GUI option makes more
sense here. You can still obtain the GUI by doing ` brew install
bitcoind --with-gui `.

Other changes:

* New recommended dependency on gettext when you build the GUI. This is
to support the translations. You can opt out of that in the usual
manner.

* Better sandboxing of the Berkeley-db4 element to avoid any accidental
weird linkage. None has been reported, but better safe than sorry, etc.
End users won’t notice this change at all.

* Moves the flags to more Homebrew-Native ENV changes instead. Looks
cleaner, and better consistency this way.

* ` bin.write_exec_script ` - This now exists because we’ve installed
bitcoind into libexec, and consequently we script things to the bin so
people can find them in the $PATH. Better sandboxing is the goal there.