BugFix: improve c++ std space import.

[suzp1984]

The namespace std is imported to srs_app_utility.cpp, but the string declaration is not inconsistent, some part use std::string, while other part use string.

And also the ::getenv(), makes me confused at the beginning.

The commit deletes the using namespace std;, I remember there is a coding convention that said to avoid importing the whole namespace in C++. What do you guys think about it?

[winlinvip]

Thank you, we will take a look.