meson: terminfo install location now defaults to $datadir/terminfo

The meson command line option -Dcustom-terminfo-install-location has
been changed in the following ways:

* If unset, $datadir/terminfo is used, and TERMINFO is *not* exported
* If set, that value (relative to $prefix) is used, and TERMINFO *is*
exported.
* The special value ‘no’ is removed.

-Ddefault-terminfo now also changes the terminfo names generated when
-Dterminfo=enabled.

Furthermore, the documentation for the TERMINFO environment variable
has been removed from the foot.1 and footclient.1 man pages (but as
mentioned above, foot *will* still set it if
-Dcustom-terminfo-install-location has been used).

INSTALL.md has been updated to now recommend using ncurses’ terminfo
definitions, if available. But also to document the other
alternatives; installing the terminfo definitions in a custom
location, or installing them with a diferent name. It also describes
the general problem, and the disadvantages of each alternative (but
without going into too much depth).

meson_options.txt

@@ -11,5 +11,5 @@ option('terminfo', type: 'feature', value: 'enabled', description: 'Build and in
 option('default-terminfo', type: 'string', value: 'foot',
        description: 'Default value of the "term" option in foot.ini.')
 
-option('custom-terminfo-install-location', type: 'string',
-       description: 'Path to foot\'s terminfo, relative to ${prefix}. If set to anything but “no“, foot will set TERMINFO to this value in the client process.')
+option('custom-terminfo-install-location', type: 'string', value: '',
+       description: 'Path to foot\'s terminfo, relative to ${prefix}. If set, foot will set $TERMINFO to this value in the client process.')