userspace code test: ncurses examples

This is the examples from

http://tldp.org/HOWTO/NCURSES-Programming-HOWTO/intro.html
ftp/display: delete dbg window
ncurses example: print F2
ftp/display: use move_cursor() to calculate the cursor pos
ftp/display: fix bug for display when input more than the command size

This patch fix two bugs.

1. initialize the command buffer to 0 at the beginning of get_command()
2. alway move the i index, but just when the i index is in the command buffer
range, the command buffer would be changed. otherwise the display would be odd
when the input is more than commnd size and the content would be not
consistent with what displayed.
ftp/command: add the command handle framework
ftp/command: print help when command is not supported