Dynamic library code improvements

I added 2 changes to the library code.

The first is the addition of #ifdef LIB_OPEN_DIRECTLY (to be defined in q_arch.h) for systems that load libraries in a way different from Windows and Linux. Android, for example, has all libraries in a system directory unpacked from the .apk during the installation.

Another modification is making Sys_Library_ErrorString const. I get a compiler warning when the output of dlerror is implicitly casted to non-const char*.