libblkid: add microsecond resolution for cache entries

libblkid: add microsecond resolution for cache entries

The libblkid library uses stat.st_mtine to detect changes on the
device. The last update time of of the device in the cache is stored
as TIME= tag in the /etc/blkid.tab file.

Linux since 2.5.48 supports nanosecond resolution and more precise
time is available in the stat.st_mtim timespec struct.

This patch add microsecond precision to TIME= tag in the cache file,
old format:

	TIME="<sec>"

the new format:

	TIME="<sec>.<usec>"

This change is backwardly compatible.

Now, the blkid_verify() function checks stat.st_mtime and
stat.st_mtim.tv_nsec/1000.

Test:

 # e2label /dev/sdb1 AAAA

old version:

 # blkid -s LABEL /dev/sdb1; e2label /dev/sdb1 BBBB; blkid -s LABEL /dev/sdb1
 /dev/sdb1: LABEL="AAAA"
 /dev/sdb1: LABEL="AAAA"

new version:

 # blkid -s LABEL /dev/sdb1; e2label /dev/sdb1 BBBB; blkid -s LABEL /dev/sdb1
 /dev/sdb1: LABEL="AAAA"
 /dev/sdb1: LABEL="BBBB"

Signed-off-by: Karel Zak <kzak@redhat.com>