Logging now doesn't lock with each message

The original logging routines were locking a common mutex. This led to
massive underperformance and unwanted serialization when heavily logging
due to lock contention. Now the logging is lockless, though still
serializing on write() syscalls to the same filedescriptor.

This change also brings in a persistent logging channel structures and
thus avoids writing into active configuration data structures during
regular run.

sysdep/unix/unix.h

@@ -120,7 +120,9 @@ enum rf_mode {
 };
 
 struct rfile *rf_open(struct pool *, const char *name, enum rf_mode mode);
-int rf_fileno(struct rfile *f);
+off_t rf_size(struct rfile *);
+int rf_same(struct rfile *, struct rfile *);
+void rf_write(struct rfile *, const void *, size_t);
 
 extern struct rfile rf_stderr;
 
@@ -138,10 +140,9 @@ void log_switch(int initial, list *l, const char *);
 struct log_config {
   node n;
   uint mask;				/* Classes to log */
-  struct rfile *rf;			/* Resource for log file; NULL=syslog */
+  struct rfile *rf;			/* File handle */
   const char *filename;			/* Log filename */
   const char *backup;			/* Secondary filename (for log rotation) */
-  off_t pos;				/* Position/size of current log */
   off_t limit;				/* Log size limit */
   int terminal_flag;
 };