adb: make adb_thread_func_t return void, add adb_thread_exit.

Windows restricts the return value of threads to 32-bits, even on 64-bit
platforms. Since we don't actually return meaningful values from thread,
resolve this inconsistency with POSIX by making adb's thread abstraction
only take void functions.

Change-Id: I5c23b4432314f13bf16d606fd5e6b6b7b6ef98b5
diff --git a/services.cpp b/services.cpp
index 9cbf787..75cbe5d 100644
--- a/services.cpp
+++ b/services.cpp
@@ -57,13 +57,11 @@
     void *cookie;
 };
 
-void *service_bootstrap_func(void *x)
-{
+static void service_bootstrap_func(void* x) {
     stinfo* sti = reinterpret_cast<stinfo*>(x);
     adb_thread_setname(android::base::StringPrintf("service %d", sti->fd));
     sti->func(sti->fd, sti->cookie);
     free(sti);
-    return 0;
 }
 
 #if !ADB_HOST