bin/sleep: add support for units other than seconds

The coreutils version of this command accepts a unit designation of s,
m, h, or d (for seconds, minutes, hours, days) immediately following
the number of (fractional) units to delay.

The submitted patch has been modified in one detail: the test meant to
detect the presence of the unit modified was not specific (!= 1) and
would have accepted a non-numeric initial element or extra characters
following the union. The committed version accepts only the number
immediately followed by one of the defined unit designators and no
further characters.

PR: 264162
MFC after: 1 week