* configure.in: Move code that provides the --enable-tui option

* configure.in: Move code that provides the --enable-tui option
before the "Checks for libraries" section.  Polish the code
somewhat and set need_curses to yes if we build the TUI.  Rewrite
code that looks for a library providing termcap functionality to
match more closely what's done in the Readline library, and move
it into to the "Checks for libraries" section.
* configure: Regenerated.
* Makefile.in (TERMCAP): Remove variable.
* config/i386/go32.mh (TERMCAP): Remove variable.