[RFAv3 0/6] Implement | (pipe) command.

[Philippe Waroquiers <philippe dot waroquiers at skynet dot 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 is the third version, handling the comments of Pedro.
The doc, help and NEWS are changed in this version, so must be
re-reviewed.

* Comments of Pedro:
  * Implement -d SEP (SEP being a string) instead of -dX.
  * popen has been kept as libiberty pexecute still implies to use
    the WIF* macros.  However, exit status handling reworked to go via
    convenience variables, rather than being shown to the user.
  * various small changes (== '\0', Skip, ...).
  * simplified error messages to just indicate with a static string
    what argument is wrong or missing.
  * extended the test to verify all error handling messages and check
    the new convenience variables $_shell_exitcode and $_shell_exitsignal.

* 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 -d SEP option allows to specify an
     alternate string to use SEP 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.