Refactor: Avoid `std::endl` where it's not necessary (part 2)

The `std::endl` manipulator, except inserting `\n` character, also
performs `os.flush()`, which may lead to undesired effects (like
disk I/O in the middle of forming data strings). For the
`std::stringstream` it also has no meaning.

* replace multiple `operator<<` calls on a string literal w/
  the only call and the only (bigger) string literal;
* replace one character string literal used in `operator<<`
  w/ a char literal.

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