Add support for GRUB2 submenus.

Properly parse the 'submenu' directive in GRUB's configuration file
and provide the user with an appropriate menu interface when choosing
which entry to reboot into.

Limitations: Internally we make use of the " >> " separator (hardcoded
in both kdm and ksmserver). An occurence of the separator in a menu
title (be it menuentry or submenu) will baffle our parser. Moreover,
we only support multi-line statements: one-liners, although supported
by GRUB, will fail to be parsed. Lastly, only up to 5 nesting levels
are supported. The above limits are more than enough for all but a
handful of cases.

(forward ported from commit b8e50674)

BUG: 297209
FIXED-IN: 4.9.1
REVIEW: 105563