Mixed log output files

[fbrausse]

In relation to #655 and #664, I see a fundamental problem with the mixed usage of stdout and stderr in ESBMC at the moment.

We currently have 2 conflicting conventions:

1. Mixed output files: stdout used for non-error messages in ESBMC, stderr for error messages. Thus no single file ESBMC output to at the moment constitutes a "log" of all actions and situations encountered. Error messages usually precede ESBMC aborting with an error exit code due to either an internal bug or wrong usage.
2. ESBMC uses/can use a lot of libraries; Clang seems to be the only one of which that makes assumptions about stderr - the same convention used by many compilers and other software (e.g. FFmpeg) potentially creating machine-readable¹ output: stderr for text to be interpreted by humans, leaving stdout to be piped to other programs or redirected.

The convention in 2. is widely adopted. Question here is, does it also apply to ESBMC? I would argue that since ESBMC can be instructed to create machine-readable output (SMT formulas, SMT models, potentially a machine-readable counter-example in the future), it should be possible for such output to go to stdout in conformance with 2. That in turn would imply that all of the current messages should go to stderr instead. That would at least solve the externally visible discrepancy between the files mentioned in 1. and also make Clang's diagnostics/parse tree part of the human-readable "log" output of ESBMC.

I realize that this technically does not "solve" #664, but adopting convention 2. would solve the mixed output file usage and for now also make Clang's diagnostics externally indistinguishable from ESBMC's - which is a good thing™. What do you think?

¹ "machine-readable" used in this post in the sense that a common agreed-upon and documented format for it exists

[lucasccordeiro]

Approach 2 looks good to me.

[fbrausse]

#1284 switched the log output to stderr.