Windows - Implement a custom version of snprintf

Windows needs to match the truncation behavior of C99 standard snprintf until MSVC supports snprintf.

The Windows CRT function _snprintf does not truncate strings. I have updated _wt_vsnprintf to truncate, and created a wt wrapper called _wt_snprintf.

This became an issue while testing wtperf, and I saw an issue in the config.c configuration parser.