Don't compile with -DGNU_SOURCE anymore, define doelogA() and report all errno-style errors using that + check returns for some functions

Related to https://github.com/FredBoiteux/empcd/commit/f2502ed2b9d710839cceb7fda6d7215346ed47dd
Add a set of very strict compilation flags, fixes for bugs found by these flags follow
Constant strings are a good thing
Compilation warning fixes.

- constant strings
- static functions along with prototypes
- cleanup unused variables
- don't pass global mpd var to empcd_status(),
- fix error reporting for f_* functions
- fixes a wrong '==' in handle_event() which should have been a simple '=' assignment.