base: add helpers for sending/receiving file descriptors.

Almost all of the uses of cmsg(3) in our source tree are wrong in at
least one of the following ways:

  - not aligning the cmsg buffer
  - leaking fds if more fds are received than expected
  - blindly dereferencing CMSG_DATA without checking the header
  - using CMSG_SPACE instead of CMSG_LEN for .cmsg_len
  - using CMSG_LEN instead of CMSG_SPACE for .msg_controllen
  - using a length specified in number of fds instead of bytes

Implement wrapper functions that implement this dumpster fire of an
API correctly.

Bug: http://b/122047630
Test: libbase_test32
Change-Id: I6ac34d67bbbf1bfa9727ab598248fc178ea19df9
diff --git a/base/Android.bp b/base/Android.bp
index b0181aa..38f301a 100644
--- a/base/Android.bp
+++ b/base/Android.bp
@@ -46,6 +46,7 @@
     defaults: ["libbase_cflags_defaults"],
     srcs: [
         "chrono_utils.cpp",
+        "cmsg.cpp",
         "file.cpp",
         "logging.cpp",
         "mapped_file.cpp",
@@ -85,6 +86,9 @@
                 "errors_windows.cpp",
                 "utf8.cpp",
             ],
+            exclude_srcs: [
+                "cmsg.cpp",
+            ],
             enabled: true,
         },
     },
@@ -121,6 +125,7 @@
     defaults: ["libbase_cflags_defaults"],
     host_supported: true,
     srcs: [
+        "cmsg_test.cpp",
         "endian_test.cpp",
         "errors_test.cpp",
         "file_test.cpp",