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).

foot.info

@@ -1,17 +1,17 @@
-foot|foot terminal emulator,
-    use=foot+base,
+@default_terminfo@|foot terminal emulator,
+    use=@default_terminfo@+base,
     colors#256,
     setab=\E[%?%p1%{8}%<%t4%p1%d%e%p1%{16}%<%t10%p1%{8}%-%d%e48\:5\:%p1%d%;m,
     setaf=\E[%?%p1%{8}%<%t3%p1%d%e%p1%{16}%<%t9%p1%{8}%-%d%e38\:5\:%p1%d%;m,
 
-foot-direct|foot with direct color indexing,
-    use=foot+base,
+@default_terminfo@-direct|foot with direct color indexing,
+    use=@default_terminfo@+base,
     colors#16777216,
     RGB,
     setab=\E[%?%p1%{8}%<%t4%p1%d%e48\:2\:\:%p1%{65536}%/%d\:%p1%{256}%/%{255}%&%d\:%p1%{255}%&%d%;m,
     setaf=\E[%?%p1%{8}%<%t3%p1%d%e38\:2\:\:%p1%{65536}%/%d\:%p1%{256}%/%{255}%&%d\:%p1%{255}%&%d%;m,
 
-foot+base|foot base fragment,
+@default_terminfo@+base|foot base fragment,
     am,
     bce,
     bw,