Multiboot with Grub 2. Do it the hard way ;-)

Upd1. Various small fixes.
Upd2. Change layout.

Table of content:
    A. Notes about grub-install.
    B. Notes about update-grub.
    C. Other notes.
    D. Multiboot schemes.
    E. Scripts and config examples.

Do not mount LV by UUID from fstab.

I should not reference LV in fstab by UUID, because if i have snapshot of this
LV, it is unknown which one (origin or snapshot) will be mounted.