livecd-iso-to-disk & editliveos: Preserve extra-kernel-args from source. Refresh vmlinuz & initrd.img upon kernel updates.

[FGrose]

1. Preserve extra-kernel-args from source

Preserve any extra kernel arguments that have been added by editliveos
or manually to the source boot configuration files.
Also:

• Add a bind mount to the host root filesystem for editliveos --script
  or shell so as to ease access to host files from the install chroot.
• Improve some error handling & reporting.
• Adjust some space accounting & reporting for filesystem variations.
• Enable the ext4 64bit feature if the host's SYSLINUX version >= 6.04
• Update all boot configuration files upon editliveos refresh.
• fsck filesystems upon editliveos refresh.
• Fix a --refresh-only bug in editliveos.

2. Refresh vmlinuz & initrd.img upon kernel updates

Detect when a new kernel has been installed and update the working
copies to the newer version for both the new .iso image and a
refreshable source, including a currently booted LiveOS image. (The
new versions will be active on the next boot.)

• Enhance kernel version checking in livecd-iso-to-disk to test
  for matching versions in the kernel and initial RAM filesystems.
• Mount macboot.img and copy its files rather than using dd on the
  whole image. This proved to be more robust during fsck.hfsplus.
• Copy proper image & syslinux directories on multi-image sources.
• Preserve mode, ownership, timestamps on more copy calls.
• Add some line breaks to separate transcript sections.
• Add a missing import to util.py.
• Update documentation.