Added compiler-style context for errors and warnings. Errors look like this:

$ bfs -nam needle
bfs: error: bfs -nam needle
bfs: error:     ~~~~
bfs: error: Unknown argument; did you mean -name?

and warnings look like this:

$ bfs -print -name 'needle'
bfs: warning: bfs -print -name needle
bfs: warning:            ~~~~~~~~~~~~
bfs: warning: The result of this expression is ignored.

Updated from C99 to C11

Fixed the tests when built against musl

Fixed a build error reported on Manjaro