x11/gbsddialog: add new port

gbsddialog is an implementation of bsddialog, a text-based user interface to be
used in scripts and tools, such as in the official FreeBSD installer,
bsdinstall(8). gbsddialog uses Gtk+ dialogs and widgets instead of console-based
equivalents. The objective is to reproduce the same behaviour as the original
tool, respecting the exact command-line parameters and output. gbsddialog can
then be used as a drop-in replacement to bsddialog, effectively granting the
possibility to install and configure FreeBSD entirely in graphical mode.

In addition, an implementation of Xdialog is also provided, for use with
bsdconfig(8) when invoked in graphical mode with -S or -X.

Reviewed by: jrm
Sponsored by: The FreeBSD Foundation
Pull Request: https://github.com/freebsd/freebsd-ports/pull/235