added rrd_strtoding & replaced all rrd_strtod instances with it
Added rrd_strtoding to the symbol export list
rrd_strtoding now takes error string as an argument

This commit replaces all rrd_strtoding calls with a 4 argument
version of the function and also make sure that its return value
(which shows whether the string has been converted partially,
fully or not converted at all) is checked at all the places.
Fixed some bad indentation
make sure that sscanf doesn't get too much data

%19[0-9e+-] makes sure that it takes only the characters required for
getting a double variable
Missed this instance of rrd_strtoding earlier
*endptr should be set in rrd_strtoding, not endptr

so that value is reflected in the callee function as well
Removed printfs left while debugging
Merge pull request #514 from Sweet-kid/strtod_api

Wrote api for rrd_strtod