clean up zero-byte logfile on immediate exit

Otherwise, a zero-byte log file stays behind after every call to `i3
--get-socketpath`. Also, replace "return" calls with more explicit "exit"
calls.

Before:

$ ls -ld /tmp/i3* | wc -l; \
  repeat 10 i3 --get-socketpath >/dev/null; \
  ls -ld /tmp/i3* | wc -l
1
11

Now:

$ ls -ld /tmp/i3* | wc -l; \
  repeat 10 i3 --get-socketpath >/dev/null; \
  ls -ld /tmp/i3* | wc -l
1
1

Signed-off-by: Julius Plenz <julius@plenz.com>

include/log.h

@@ -67,4 +67,11 @@ void errorlog(char *fmt, ...)
 void verboselog(char *fmt, ...)
     __attribute__ ((format (printf, 1, 2)));
 
+/**
+ * Deletes the unused log files. Useful if i3 exits immediately, eg.
+ * because --get-socketpath was called. We don't care for syscall
+ * failures. This function is invoked automatically when exiting.
+ */
+void purge_zerobyte_logfile(void);
+
 #endif