linguas: use en_US as UTF-8 locale, C.UTF-8 is not a standard locale.

Apparently some distros like Debian provide C.UTF-8 but it isn't
actually fully supported by glibc and thus is not available on all
systems, neither Gentoo nor Fedora provide it.

https://sourceware.org/bugzilla/show_bug.cgi?id=17318
gpt: start new GPT module

This module is a new implementation for reading GUID Partition Tables
which is much stricter than the existing part_gpt module and exports GPT
data directly instead of the generic grub_partition structure. It will
be the basis for modules that need to read/write/update GPT data.

The current code does nothing more than read and verify the table.
Merge pull request #1 from marineam/gptprio

Begin new GPT implementation for GRUB2