Support socket activation of adb server

Socket activation allows adb to be run as a system daemon that starts
only as needed and supports race-free transparent restarts of the adb
server after a client issues an "adb kill-server" command.

Test: see SOCKET-ACTIVATION.txt
Change-Id: Ieabf08710ce4365e5513102f3aa578560aa7355e
diff --git a/adb/client/commandline.cpp b/adb/client/commandline.cpp
index 0ffdbc2..6465ffe 100644
--- a/adb/client/commandline.cpp
+++ b/adb/client/commandline.cpp
@@ -107,6 +107,7 @@
         "       localfilesystem:<unix domain socket name>\n"
         "       dev:<character device name>\n"
         "       jdwp:<process pid> (remote only)\n"
+        "       acceptfd:<fd> (listen only)\n"
         " forward --remove LOCAL   remove specific forward socket connection\n"
         " forward --remove-all     remove all forward socket connections\n"
         " ppp TTY [PARAMETER...]   run PPP over USB\n"