basic/log: make log_{info,warning,...} return void

log_debug still returns 0. I think it is legitimate to use 'return log_debug()' to
return 0. It is different than the other functions, since we often want to supress
errors logged at debug level. This case is quite common in the codebase and
we could use 'return log_debug_errno()' to make the code more consise.

For all other variants, a separate return line is required.
Previous commit changes all the non-conforming instances, now we can make it mandatory.