Fix possible memory error with long selections

Fix possible memory error with long selections

If a selection was more than 1000 characters long and there was a
whitespace exactly at the 1000 point, a buffer overflow could occur.
Replaced the buffer with std::string, simplifying the code
significantly.  Update the generated code to use a newer flex, which
also removes the need for some suppressions.

Should fix #2086.

Change-Id: I56513bcf5ee99f05ce144461740d0f868be10186