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).

src/i3.mk

@@ -7,7 +7,7 @@ i3_SOURCES           := $(filter-out $(i3_SOURCES_GENERATED),$(wildcard src/*.c)
 i3_HEADERS_CMDPARSER := $(wildcard include/GENERATED_*.h)
 i3_HEADERS           := $(filter-out $(i3_HEADERS_CMDPARSER),$(wildcard include/*.h))
 i3_CFLAGS             = $(XCB_CFLAGS) $(XCB_KBD_CFLAGS) $(XCB_WM_CFLAGS) $(X11_CFLAGS) $(XCURSOR_CFLAGS) $(YAJL_CFLAGS) $(LIBEV_CFLAGS) $(PCRE_CFLAGS) $(LIBSN_CFLAGS)
-i3_LIBS               = $(XCB_LIBS) $(XCB_KBD_LIBS) $(XCB_WM_LIBS) $(X11_LIBS) $(XCURSOR_LIBS) $(YAJL_LIBS) $(LIBEV_LIBS) $(PCRE_LIBS) $(LIBSN_LIBS) -lm
+i3_LIBS               = $(XCB_LIBS) $(XCB_KBD_LIBS) $(XCB_WM_LIBS) $(X11_LIBS) $(XCURSOR_LIBS) $(YAJL_LIBS) $(LIBEV_LIBS) $(PCRE_LIBS) $(LIBSN_LIBS) -lm -lpthread
 
 # When using clang, we use pre-compiled headers to speed up the build. With
 # gcc, this actually makes the build slower.