issue wakeup when enter a new segment (previous was too often and not

issue wakeup when enter a new segment (previous was too often and not
often enough); fix sum_bytes_left leak in lfs_writefile; update segusage table
correctly when rewriting an inode; check space before allocation, else can
allocate unnecessarily; don't update last mod when rewriting a block; fix
segment usage information when writing the segment

SCCS-vsn: sys/ufs/lfs/lfs_segment.c 7.12