Support time units for -mtime etc.

[tavianator]

FreeBSD find supports -{B,a,c,m}time n[smhdw]:

     If	units are specified, this primary evaluates to true if the
     difference	between	the file last access time and the time find
     was started is exactly n units.  Possible time units are as fol-
     lows:

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

     Any number	of units may be	combined in one	-atime argument, for
     example, "-atime -1h30m".	Units are probably only	useful when
     used in conjunction with the + or - modifier.