Ignore setsid error in some cases.

If the calling process has already been a leading process of session.
setsid just fail with EPERM, ignore such error.

Test: killall adb;exec 3>f;adb fork-server server --reply-fd 3 & cat f
Change-Id: I1aeac079f29e10aa63ed724b5a43663f25c25ad5
Signed-off-by: Tao Wu <lepton@google.com>
diff --git a/adb/client/main.cpp b/adb/client/main.cpp
index 571c227..e428c49 100644
--- a/adb/client/main.cpp
+++ b/adb/client/main.cpp
@@ -129,7 +129,9 @@
         // Start a new session for the daemon. Do this here instead of after the fork so
         // that a ctrl-c between the "starting server" and "done starting server" messages
         // gets a chance to terminate the server.
-        if (setsid() == -1) {
+        // setsid will fail with EPERM if it's already been a lead process of new session.
+        // Ignore such error.
+        if (setsid() == -1 && errno != EPERM) {
             fatal("setsid() failed: %s", strerror(errno));
         }
 #endif