[MEDIUM] add support for time units in the configuration

[MEDIUM] add support for time units in the configuration

It is not always handy to manipulate large values exprimed
in milliseconds for timeouts. Also, some values are entered
in seconds (such as the stats refresh interval). This patch
adds support for time units. It knows about 'us', 'ms', 's',
'm', 'h', and 'd'. It automatically converts each value into
the caller's expected unit. Unit-less values are still passed
unchanged.

The unit must be passed as a suffix to the number. For instance:

     clitimeout 15m

If any character is not understood, an error is returned.