Add const to some char* strings

[enp1s0]

This pull request adds const to some char* strings.
This allows us to use this library in C++ as follows.

const std::string name = "...";
starsh_problem_new(..., name.c_str(), ...);

Otherwise, we need to remove const using const_cast and it's not safe.

const std::string name = "...";
starsh_problem_new(..., const_cast<char*>(name.c_str()), ...);