[BUG] use C++11 keyword nullptr

[LeSpocky]

Prerequisites

• Put an X between the brackets on this line if you have checked that your issue isn't already filed: https://github.com/search?l=&q=repo%3Aetr%2Flibhttpserver&type=Issues

Description

C++ from C++11 onwards has the keyword nullptr to help compiler and developer communicating intent when using null pointers. Use that instead of confusing fellow developers with unusual variants of null.

Steps to Reproduce

n.a.

Expected behavior: Use nullptr if null pointer is meant.

Actual behavior: 0x0 is used in huge parts of the codebase.

Reproduces how often: always.

Versions

• OS version: n.a.
• libhttpserver version: 0.18.2-22-g263f2bd (current master)
• libmicrohttpd version: n.a.

Additional Information

According to #227 C++14 is required for libhttpserver already, so using C++11 features should be fine.

[LeSpocky]

Somehow related to #177, did not see that one before. 😳