Add a way to turn off unique_fd's operator int.

unique_fd's implicit conversion to int has led to tons of problems (see
all of the overloads for close, fdopen, fdopendir, etc.). Add a switch
that can turn it off, and reduce the ridiculous amount of work to fix up
callers by introducing a borrowed_fd type that can be constructed from
either int or unique_fd.

Test: treehugger
Change-Id: If77cf5cbcaddacdaec5919a15b3520fb68f51a62
diff --git a/adb/socket_spec.cpp b/adb/socket_spec.cpp
index de4fff9..1333724 100644
--- a/adb/socket_spec.cpp
+++ b/adb/socket_spec.cpp
@@ -314,14 +314,14 @@
         addr.svm_port = port == 0 ? VMADDR_PORT_ANY : port;
         addr.svm_cid = VMADDR_CID_ANY;
         socklen_t addr_len = sizeof(addr);
-        if (bind(serverfd, reinterpret_cast<struct sockaddr*>(&addr), addr_len)) {
+        if (bind(serverfd.get(), reinterpret_cast<struct sockaddr*>(&addr), addr_len)) {
             return -1;
         }
-        if (listen(serverfd, 4)) {
+        if (listen(serverfd.get(), 4)) {
             return -1;
         }
         if (serverfd >= 0 && resolved_port) {
-            if (getsockname(serverfd, reinterpret_cast<sockaddr*>(&addr), &addr_len) == 0) {
+            if (getsockname(serverfd.get(), reinterpret_cast<sockaddr*>(&addr), &addr_len) == 0) {
                 *resolved_port = addr.svm_port;
             } else {
                 return -1;