Compatability of '-256color' terminal descriptions between ncurses 6.0 and ncurses 6.1

[Vladimir Lomov]

Hello,

I'm using Archlinux x86_64 and recently ncurses package was updated from
ver. 6.0+20170902-3 to ver. 6.1-3 and that makes MC (midnight-commander,
compiled vs SLANG) fail to start with following message:

```
Unknown terminal: tmux-256color
Check the TERM environment variable.
Also make sure that the terminal is defined in the terminfo database.
Alternatively, set the TERMCAP environment variable to the desired
termcap entry.
```

The same message I see if I choose other '-256color' terminal
descriptios, like 'screen-256color', 'xterm-256color' (MC starts but
warning that TD doesn't have color support), 'stterm-256color',
'konsole-256color', 'vte-256color', 'gnome-256color' and 'st-256color'.

Terminal descriptions (TERM=...) without '-256color' work fine with both
versions. Needless to say that ncurses 6.0 (as provided by my distro)
works fine (xterm-256color has color support).

I tried to compare terminal description for 'tmux-256color' from ncurses
6.0 and ncurses 6.1 but (obviously?) 'infocmp' from ncurses 6.0 can't
find TD for 'tmux-256color' while 'infocmp' from ncurses 6.1 works fine.

I did following:
```
$ mkdir -p ~/tmp/6.{0,1}/t

# [while ncurses 6.1 package is installed]
$ cp /usr/share/terminfo/t/tmux-256color ~/tmp/6.1/t

# [while ncurses 6.0 package is installed]
$ cp /usr/share/terminfo/t/tmux-256color ~/tmp/6.0/t

# [comparing using ncurses 6.0 infocmp]
$ TERM=tmux-256color infocmp -d -B ~/tmp/6.1 tmux-256color
infocmp: couldn't open terminfo file ~/tmp/6.1/t/tmux-256color.

# [reinstalling ncurses 6.1 package, reformatted output]
$ TERM=tmux-256color infocmp -d -A ~/tmp/6.0 tmux-256color
comparing tmux-256color to tmux-256color.
    comparing booleans.
    comparing numbers.
        pairs: 32767, 65536.
    comparing strings.
        hpa: NULL, '\E[%i%p1%dG'.
        indn: NULL, '\E[%p1%dS'.
        invis: NULL, '\E[8m'.
        sgr: 
'\E[0%?%p6%t;1%;%?%p1%t;3%;%?%p2%t;4%;%?%p3%t;7%;%?%p4%t;5%;%?%p5%t;2%;m%?%p9%t\016%e\017%;',
             '\E[0%?%p6%t;1%;%?%p1%t;7%;%?%p2%t;4%;%?%p3%t;7%;%?%p4%t; 
5%;%?%p5%t;2%;m%?%p9%t\016%e\017%;'.
        vpa: NULL, '\E[%i%p1%dd'.

# [other check]
$ TERM=tmux-256color infocmp -d -A ~/tmp/6.1 tmux-256color
comparing tmux-256color to tmux-256color.
    comparing booleans.
    comparing numbers.
    comparing strings.
```

Is it expected behaviour (infocmp from 6.0 can't find TD from 6.1)? Is
it expected that '-256color' TDs are not compatible?

I already submitted bug report for my distro but I'm not sure is it a
packaging (building) or upstream bug.

---
WBR, Vladimir Lomov

-- 
Tuesday After Lunch is the cosmic time of the week.