Enable buffering for parsing agent output (bnc#854809).

Apparently reading 1 character at a time forces the flex scanner to
spend quadratic time on each token, in yy_get_previous_state.
For one sudoers file we had large tokens, one of them 380KB, taking 14
minutes to parse.

In hindsight, buffering should be enabled by default, turning it off for
specific cases of interactive use.
version + changelog
Merge pull request #84 from mvidner/sudoers-buffering2

Enable buffering for parsing agent output (bnc#854809)