LU-12027 utils: add units to "lfs find -amctime"

LU-12027 utils: add units to "lfs find -amctime"

The normal find command can only specify time arguments in terms
of whole days. The MacOS find(1) man page reports the use of unit
suffixes to give better control over the time range, such as "1d4h".

Possible time units are as follows:

    s       second
    m       minute (60 seconds)
    h       hour (60 minutes)
    d       day (24 hours)
    w       week (7 days)
    y       year (365 days)

There is no "month" specifier here or in find(1), since the length
of a month is not fixed, and it would conflict with the 'm' minutes
unit.  Units may be combined in one argument, e.g. "-atime -1h30m".

For matches that are "equal" to the specified time, they must match
within the smallest unit specified.  For example, "-mtime 24h" would
match anything within 1h of 24h ago, similar to "-size 100M" will
match anything within 1MB of 100MB.

Test-Parameters: trivial fstype=zfs
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: Ib1eb4e626b712bb75f13b075849f959f003ebbe5
Reviewed-on: https://review.whamcloud.com/34367
Reviewed-by: Wang Shilong <wshilong@ddn.com>
Reviewed-by: Li Xi <lixi@ddn.com>
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>