Define internal vget_string helper function

[rillig]

Instead of messing around with the get_string macro, and having one definition inside cs50.c and another definition everywhere else, the cs50 code should demonstrate good knowledge of the C programming language and its larger ecosystem by implementing get_string in the same way as the C Standard Library implements fprintf:

• there's fprintf, which takes varargs and behaves as if it calls vfprintf internally.
• there's vfprintf, which takes a va_list as argument.

This makes the code much cleaner and gets rid of the confusing macro. Remember, your students will look at that code and will use it as a role model, and if they later learn that your code style is bad, they will mistrust you in everything else you taught them, and rightly so.

[rillig]

When trying out libcs50 in JetBrains CLion, the definition of the get_string macro produces this completion:

This is unnecessarily confusing and should be fixed by not needing the macro at all.