Poco::Thread::current() is not reliable for logging

It can return null for no obvious reason, leading to misleading
logging where the same thread is identified as numer zero at one place
and non-zero at another. So use the actual Linux thread id in logging.

Sure, thread ids are somewhat less convenient, as they are larger
numbers, from the same number space as process ids.

loolwsd/Log.cpp

@@ -15,6 +15,9 @@
 #include <sstream>
 #include <string>
 
+#include <sys/syscall.h>
+#include <unistd.h>
+
 #include <Poco/ConsoleChannel.h>
 #include <Poco/FileChannel.h>
 #include <Poco/FormattingChannel.h>
@@ -88,8 +91,8 @@ namespace Log
         const char *appName = (Source.inited ? Source.id.c_str() : "<shutdown>");
         assert(strlen(appName) + 32 + 28 < 1024 - 1);
 
-        snprintf(buffer, 4095, "%s-%.2d %d:%.2d:%.2d.%.6d [ %s ] %s  ", appName,
-                 (Poco::Thread::current() ? Poco::Thread::current()->id() : 0),
+        snprintf(buffer, 4095, "%s-%.04lu %d:%.2d:%.2d.%.6d [ %s ] %s  ", appName,
+                 syscall(SYS_gettid),
                  (int)hours, (int)minutes, (int)seconds, (int)usec,
                  procName, level);
     }