Make transform*, --info-command, and execute-silent cancellable

Users can press CTRL-C after 1 second to terminate the command.

Close #3883

man/man1/fzf.1

@@ -457,6 +457,8 @@ Determines the display style of the finder info. (e.g. match counter, loading in
 Command to generate the finder info line. The command runs synchronously and
 blocks the UI until completion, so make sure that it's fast. ANSI color codes
 are supported. \fB$FZF_INFO\fR variable is set to the original info text.
+For additional environment variables available to the command, see the section
+ENVIRONMENT VARIABLES EXPORTED TO CHILD PROCESSES.
 
 e.g.
      \fB# Prepend the current cursor position in yellow