Replace non-threadsafe strerror

Log the name of the error as well as the error code if a network problem
happens. This makes network troubleshooting more convenient.

Use thread-safe strerror_r and the WIN32 equivalent FormatMessage.
Replace non-threadsafe gmtime and setlocale

Make DateTimeStrFormat use boost::posix_time.

Also re-enable the util_DateTimeStrFormat tests, as they are no
longer platform specific.