BUG/MINOR: tools: make parse_time_err() more strict on the timer validity

BUG/MINOR: tools: make parse_time_err() more strict on the timer validity

First, an error is now reported if the first character is not a digit. Thus,
"timeout client s" triggers an error now. Then 'u' is also rejected
now. 'us' is valid and should be used set the timer in microseconds. However
'u' alone is not a valid unit. It was just ignored before (default to
milliseconds). Now, it is an error. Finally, a warning is reported if the
end of the text is not reached after the timer parsing. This warning will
probably be switched to an error in a futur version.

This patch must be backported to all stable versions.

(cherry picked from commit c20ad0d8dbd1bb5707bbfe23632415c3062e046c)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>