Enhance line discipline in PipeCapture::OnReadable() (#709276)

Add concept of cursor position within the current line, separate from
the end of the buffer.  This is so that programs which output a text
progress bar using backspace, such as resize2fs -p, are displayed
correctly.

Bug #709276 - Percentage indicator for subcommand
Turn on resize2fs text progress bar (#709276)

Turn on the text progress bar for the resize2fs command.  Looks like:

    # resize2fs -p /dev/sda13 262144
    resize2fs 1.41.12 (17-May-2010)
    Resizing the filesystem on /dev/sda13 to 262144 (4k) blocks.
    Begin pass 2 (max = 97177)
    Relocating blocks             XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
    Begin pass 3 (max = 80)
    Scanning inode table          XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
    Begin pass 4 (max = 5597)
    Updating inode references     XXXXXXXXX-------------------------------

Bug #709276 - Percentage indicator for subcommand