Implement | (pipe) command.

Implement | (pipe) command.

The pipe command allows to run a GDB command, and pipe its output
to a shell command:
  (gdb) help pipe
  Send the output of a gdb command to a shell command.
  Usage: | [COMMAND] | SHELL_COMMAND
  Usage: | -d DELIM COMMAND DELIM SHELL_COMMAND
  Usage: pipe [COMMAND] | SHELL_COMMAND
  Usage: pipe -d DELIM COMMAND DELIM SHELL_COMMAND

  Executes COMMAND and sends its output to SHELL_COMMAND.

  The -d option indicates to use the string DELIM to separate COMMAND
  from SHELL_COMMAND, in alternative to |.  This is useful in
  case COMMAND contains a | character.

  With no COMMAND, repeat the last executed command
  and send its output to SHELL_COMMAND.
  (gdb)

For example:
  (gdb) pipe print some_data_structure | grep -B3 -A3 something

The pipe character is defined as an alias for pipe command, so that
the above can be typed as:
  (gdb) | print some_data_structure | grep -B3 -A3 something

If no GDB COMMAND is given, then the previous command is relaunched,
and its output is sent to the given SHELL_COMMAND.

This also defines convenience vars $_shell_exitcode and $_shell_exitsignal
to record the exit code and exit signal of the last shell command
launched by GDB e.g. by "shell", "pipe", ...

gdb/ChangeLog
2019-05-31  Philippe Waroquiers  <philippe.waroquiers@skynet.be>

	* cli/cli-cmds.c (pipe_command): New function.
	(_initialize_cli_cmds): Call add_com for pipe_command.
	Define | as an alias for pipe.
	(exit_status_set_internal_vars): New function.
	(shell_escape): Call exit_status_set_internal_vars.
	cli/cli-decode.c (find_command_name_length): Recognize | as
	a single character command.