Allow the adb server to bind on ::1

Currently the adb server can only bind on 127.0.0.1 or * if -a is
provided. This diff adds the ability for adb to bind on ::1 as
well for use cases where you might want to force adb to be IPv6
only.

To bind the adb server on ::1
```
$ ./adb -L tcp:[::1]:1234 server
$ lsof -nPi :1234
COMMAND ... NODE NAME
adb     ...  TCP [::1]:1234 (LISTEN)
```
The original behaviour is also retained, so this would only affect
users explicitly specifying ::1 in the socket spec
```
$ export ANDROID_ADB_SERVER_PORT=1234
$ ./adb server
$ lsof -nPi :1234
COMMAND ... NODE NAME
adb     ...  TCP 127.0.0.1:1234 (LISTEN)
```

Note: I've only implemented this behaviour for posix base systems,
due to my limited understanding of networking on windows. If
needed I can do some research to implement it there as well, but
there is currently no IPv6 support at all on the windows side.

Test: New unit test and the commands in the summary

Change-Id: I23e4531e8dfda4de9348124ad7491d728aecdbf7
diff --git a/sysdeps/network.h b/sysdeps/network.h
index 83ce371..fadd155 100644
--- a/sysdeps/network.h
+++ b/sysdeps/network.h
@@ -19,4 +19,4 @@
 #include <string>
 
 int network_loopback_client(int port, int type, std::string* error);
-int network_loopback_server(int port, int type, std::string* error);
+int network_loopback_server(int port, int type, std::string* error, bool prefer_ipv4);