libfdisk: (gpt) add API for raw partition attributes

libfdisk: (gpt) add API for raw partition attributes

The existing APIs can only toggle individual bits or get and set bits in
a complex text representation, making it impractical to use libfdisk for
manipulating the GPT partition attribute field in more complex ways such
as updating a value that is multiple bits wide. For example priority
based partition selection originally designed for ChromeOS includes two
integer values that are 4-bits wide.

http://www.chromium.org/chromium-os/chromiumos-design-docs/disk-format#TOC-Trusting-the-GPT

[kzak@redhat.com: - add new symbols to .sym file and docs
                  - remove unused variables from test program]

Signed-off-by: Michael Marineau <michael.marineau@coreos.com>
Signed-off-by: Karel Zak <kzak@redhat.com>