src/tool: allow timeouts to accept decimal values

Implement wrappers around strtod to convert the user argument to a
double with sane error checking. Use this to allow --max-time and
--connect-timeout to accept decimal values instead of strictly integers.

The manpage is updated to make mention of this feature and,
additionally, forewarn that the actual timeout of the operation can
vary in its precision (particularly as the value increases in its
decimal precision).