Add a general way to request an immediate top-level callback.

This is a little like schedule_timer, in that the callback you provide
will be run from the top-level message loop of whatever application
you're in; but unlike the timer mechanism, it will happen
_immediately_.

The aim is to provide a general way to avoid re-entrance of code, in
cases where just _doing_ the thing you want done is liable to trigger
a confusing recursive call to the function in which you came to the
decision to do it; instead, you just request a top-level callback at
the message loop's earliest convenience, and do it then.

[originally from svn r10019]

Recipe

@@ -288,7 +288,7 @@ GUITERM  = TERMINAL window windlg winctrls sizetip winucs winprint
          + winutils wincfg sercfg winhelp winjump
 
 # Same thing on Unix.
-UXTERM   = TERMINAL uxcfg sercfg uxucs uxprint timing
+UXTERM   = TERMINAL uxcfg sercfg uxucs uxprint timing callback
 GTKTERM  = UXTERM gtkwin gtkcfg gtkdlg gtkfont gtkcols xkeysym
 OSXTERM  = UXTERM osxwin osxdlg osxctrls
 
@@ -308,7 +308,7 @@ SFTP     = sftp int64 logging
 
 # Miscellaneous objects appearing in all the network utilities (not
 # Pageant or PuTTYgen).
-MISC     = timing misc version settings tree234 proxy conf
+MISC     = timing callback misc version settings tree234 proxy conf
 WINMISC  = MISC winstore winnet winhandl cmdline windefs winmisc winproxy
          + wintime
 UXMISC   = MISC uxstore uxsel uxnet cmdline uxmisc uxproxy time