unnamed threads: Obliterate

Ever read gdb output and you just kinda get a headache because you have
to infer what a thread is by reading the stack trace? It's not hard, but
we could also just never have to do that again, which is also not hard.

Sample:

(gdb) info thr
  Id   Target Id                    Frame
* 1    LWP 3719283 "nix-daemon"     0x00007e558587da0f in accept ()
   from target:/nix/store/c10zhkbp6jmyh0xc5kd123ga8yy2p4hk-glibc-2.39-52/lib/libc.so.6
  2    LWP 3719284 "signal handler" 0x00007e55857b2bea in sigtimedwait ()
   from target:/nix/store/c10zhkbp6jmyh0xc5kd123ga8yy2p4hk-glibc-2.39-52/lib/libc.so.6

The API design for this is forced by the macOS pthread_setname_np only
being able to change the current thread's name, but if we just conform
everything to that, it works everywhere.

Change-Id: I2b1d6ed41e3c94170cb0b4e73ad66f239ebd9c88

src/libutil/thread-name.cc

@@ -0,0 +1,20 @@
+#include <pthread.h>
+#if defined(__FreeBSD__) || defined(__OpenBSD__)
+#include <pthread_np.h>
+#endif
+
+namespace nix {
+
+void setCurrentThreadName(const char * name)
+{
+    // https://stackoverflow.com/questions/2369738/how-to-set-the-name-of-a-thread-in-linux-pthreads/7989973
+#if defined(__linux__)
+    pthread_setname_np(pthread_self(), name);
+#elif defined(__APPLE__)
+    pthread_setname_np(name);
+#elif defined(__FreeBSD__) || defined(__OpenBSD__)
+    pthread_set_name_np(pthread_self(), name);
+#endif
+}
+
+}