nilfs-utils: renew parser library

This renames checkpoint number library "lib/cno.c" to "lib/parser.c",
and moves a parser routine of a protection period option included in
nilfs-clean.c into parser.c to make it available from other programs.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
lssu: add option to print count of live blocks

This adds new options "-l" and "-p" to lssu command.

"-l" option prints count and ratio of live blocks for in-use segments,
and "-p" option allows users to specify a protection period which is
used to judge whether blocks are live or not.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>