If terminfo could not be found, set Reline::Terminfo.enabled == false

Fix ruby#447 ruby#543

This problem occurs when Fiddle can be loaded, curses can be loaded, and TERM is not registered in Terminfo.
It should also occur at hardcopy terminals and when Terminfo information is low, but no such reports have been received.

Reline should not abort the process because of missing Terminfo.
Reline proceeds with `Reline::Terminfo.enabled == false` when fiddle or curses cannot be loaded.
https://github.com/ruby/reline/blob/ebab2875f1226f877376558d8758bc0e2a1776c7/lib/reline/terminfo.rb#L156-L160

And does the same when Terminfo is present but TERM is not.

You can check the operation with `TERM=foo bundle exec bin/console`.

lib/reline/terminfo.rb

@@ -80,23 +80,11 @@ module Reline::Terminfo
   def self.setupterm(term, fildes)
     errret_int = Fiddle::Pointer.malloc(Fiddle::SIZEOF_INT)
     ret = @setupterm.(term, fildes, errret_int)
-    errret = errret_int[0, Fiddle::SIZEOF_INT].unpack1('i')
     case ret
     when 0 # OK
-      0
+      @enabled = true
     when -1 # ERR
-      case errret
-      when 1
-        raise TerminfoError.new('The terminal is hardcopy, cannot be used for curses applications.')
-      when 0
-        raise TerminfoError.new('The terminal could not be found, or that it is a generic type, having too little information for curses applications to run.')
-      when -1
-        raise TerminfoError.new('The terminfo database could not be found.')
-      else # unknown
-        -1
-      end
-    else # unknown
-      -2
+      @enabled = false
     end
   end
 
@@ -149,7 +137,7 @@ def self.tigetnum(capname)
   end
 
   def self.enabled?
-    true
+    @enabled
   end
 end if Reline::Terminfo.curses_dl