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.

initramfs and noexec /tmp

If during creation of new initramfs by update-initramfs (called by e.g.
kernel postinst script) /tmp is mounted with 'noexec', initramfs will be
slightly different and may report errors, like

E: /scripts/local-premount/resume failed with return 255.

shell's 'errexit' option and variable definition.

Shell's option 'errexit' works unexpectedly when command returning non-zero
is called in `local` variable definition statement.

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.

xargs, arguments separator and escapes.

xargs uses blanks (not only newlines!) to split input line into arguments.
Though, there is some options, which turns this defailt off implicitly or
explicitly.

Here is summary:

    +--------+--------------------+---------------------+
    | Option | Argument separator | Escapes             |
    +--------+--------------------+---------------------+
    | -L     | blank              | Interpreted (work)  |
    +--------+--------------------+---------------------+
    | -d     | user defined       | Literal (not work)  | 
    +--------+--------------------+---------------------+
    | -I     | newline            | Interpreted (work)  |
    +--------+--------------------+---------------------+