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.

Conflicts:
	src/netbase.cpp

Rebased-By: Wladimir van der Laan <laanwj@gmail.com>
Rebased-From: a60838d
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.

Rebased-By: Wladimir J. van der Laan <laanwj@gmail.com>
Rebased-From: 3e8ac6a