Add build status

(closes #1)
Remove branch specifier
README.rst: add libbsd to dependency list
CmakeLists: use CMAKE_C_FLAGS when setting it again

Fixes support for the CFLAGS env var
use `const char *` in parse_<type>
Initialize arguments struct's char* to zero
Repack arguments structure

Thanks @HalosGhost.
CMakeLists: Enable PIE, full RELRO and stack protector