[RFAv2 0/6] Implement | (pipe) command.

[Philippe Waroquiers <philippe.waroquiers@skynet.be>]

Implement | (pipe) command.

This patch series adds the pipe command, that allows to send the output
of a GDB command to a shell command.

This version handles the comments received.
As there is a new option -dX to the pipe_command, the doc and help
was changed to describe it.

* Comments from Eli :
   - better definition of WIF* macros for MinGW.
   - replace @ref by @xref in the doc.

* Comment from Abhijit Halder/Tom:
   it not that unlikely to have | in a GDB command
   => an optional -dX option allows to specify an
   alternate character to use X to replace the | as separator
   between the GDB COMMAND and the SHELL_COMMAND.

* Comments from Tom:
  * make previous_saved_command_line static. For this, saved_command_line
    is now also static, and all repeat related functions/vars are now
    in top.c
  * various small changes (use std::swap, strchr, .empty (), ...).
  * removed the scoped_restore_current_thread restore
  * popen has been kept as libiberty pexecute still implies to use
    the WIF* macros.
  * Instead of using execute_command_to_string, use GDB redirection
    mechanism.  I did several trials for this, and at the end,
    the only one working properly was very close to the code
    of execute_command_to_string.
    => we now have a function execute_command_to_ui_file that is used
    to implement the pipe command, and also used by
    execute_command_to_string.