shm-logging: implement i3-dump-log -f (follow)

This changes the SHM log format, it doesn’t use 0-bytes to separate
entries anymore. Instead of using lots of printf() calls in i3-dump-log,
we now do precisely one big write().

So, to be clear: i3-dump-log and i3 both need to be upgraded.
Mismatching versions will lead to garbage output (no crashes of i3, just
garbage output).

The -f flag uses an inter-process pthread_cond_t in the shared memory
header to broadcast the arrival of new messages to all i3-dump-log
processes. This internally uses futexes and thus doesn’t even mean a
kernel call in most cases. inter-process pthread_cond_ts require NPTL
(the Native Posix Thread Library, introduce in Linux 2.6).

include/shmlog.h

@@ -12,11 +12,33 @@
 #define _I3_SHMLOG_H
 
 #include <stdint.h>
+#include <pthread.h>
 
+/*
+ * Header of the shmlog file. Used by i3/src/log.c and i3/i3-dump-log/main.c.
+ *
+ */
 typedef struct i3_shmlog_header {
+    /* Byte offset where the next line will be written to. */
     uint32_t offset_next_write;
+
+    /* Byte offset where the last wrap occured. */
     uint32_t offset_last_wrap;
+
+    /* The size of the logfile in bytes. Since the size is limited to 25 MiB
+     * an uint32_t is sufficient. */
     uint32_t size;
+
+    /* wrap counter. We need it to reliably signal to clients that we just
+     * wrapped (clients cannot use offset_last_wrap because that might
+     * coincidentally be exactly the same as previously). Overflows can happen
+     * and don’t matter — clients use an equality check (==). */
+    uint32_t wrap_count;
+
+    /* pthread condvar which will be broadcasted whenever there is a new
+     * message in the log. i3-dump-log uses this to implement -f (follow, like
+     * tail -f) in an efficient way. */
+    pthread_cond_t condvar;
 } i3_shmlog_header;
 
 #endif