Add bidirectional redirection to shell command execution function (Ctrl-t R and --exec option)

The "shell command execution feature" previously only supported the
ability to transfer a command's stdout and stderr to a device.

To support bidirectional commands, a feature will be added to connect
input from a device to the command's stdin.

Since some communication commands (sz, rz) require stderr to be kept
local, a feature will also be added to not transfer stderr to a device
if the command string begins with a '?'.

On Linux, you can terminate a running command by pressing Ctrl-t R again
while the command is running (this uses the command /usr/bin/pkill
internally).

src/misc.h

@@ -38,3 +38,7 @@ double get_current_time(void);
 bool match_patterns(const char *string, const char *patterns);
 int execute_shell_command(int fd, const char *command);
 void clear_line();
+
+#if defined(__linux__)
+void terminate_shell_command(void);
+#endif