A bunch of doc and other small fixes

This patch adds a couple of both externally and internally reported
issues:
  - missing SGML tags (Issue 54), report and patch by superdupont
  - wrong variable used in the init.d script, report and patch by
    Karsten Keil <karsten-keil@t-online.de>
  - man page for gnt-instance reinstall needs clarification (Issue 56)
  - gnt-instance man page missing --disks documentation for
    replace-disks
  - gnt-node modify help output is unclear about the -C/-D/-O input
    format, and the man page doesn't document this command at all
  - “gnt-node modify -C yes” for offline or drained nodes had wrong
    error message
  - “gnt-instance reinstall --select-os” has wrong prompt, we only
    accept a number for the OS and not the template name

Reviewed-by: ultrotter