lib: [strutils] avoid integer overflow on large values

lib: [strutils] avoid integer overflow on large values

This is visible on a 2TB disk via lsblk, where a large partition
incorrectly displays as 1.171798692T instead of 1.8T. This is
corrected by using a uint64_t type instead of a simple int --
consistant with the type used in lsblk.c to represent the raw size in
bytes.

[kzak@redhat.com: - add EiB support
                  - add size_to_human_string() to regression tests]

Signed-off-by: Dave Reisner <d@falconindy.com>
Signed-off-by: Karel Zak <kzak@redhat.com>