Remove 'cb' parameter and modify fhandler_* constructors throughout.

Remove 'cb' parameter and modify fhandler_* constructors throughout.
* dtable.cc (dtable::build_fhandler): Remove debugging output which uses 'cb'.
* exec.cc (execvp): New function.
(execvpe): Ditto.
* fhandler.cc (fhandler_base::fhandler_base): Use constructor initialization.
* fhandler.h (fhandler_tty_common::fhandler_tty_common): Ditto.
* fhandler_clipboard.cc (fhandler_dev_clipboard::fhandler_dev_clipboard):
Ditto.
* fhandler_console.cc (fhandler_console::fhandler_console): Ditto.
* fhandler_raw.cc (fhandler_dev_raw::fhandler_dev_raw): Ditto.
* fhandler_serial.cc (fhandler_serial::fhandler_serial): Ditto.
* fhandler_tty.cc (fhandler_tty_master::fhandler_tty_master): Ditto.
(fhandler_tty_slave::fhandler_tty_slave): Ditto.
(fhandler_pty_master::fhandler_pty_master): Ditto.
* fhandler_windows.cc (fhandler_windows::fhandler_windows): Ditto.