mount.nilfs2: ensure gcpid option is dropped if the value is zero

Legacy version of mount.nilfs2 and umount.nilfs2 may leave invalid
mount option string "gcpid=0" in /etc/mtab.  This adds a new macro
replace_drop_opt() which replaces or drops mount option based on the
given condition, and uses the macro to ensure the gcpid option will be
deleted when its value equals zero.

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