adb was already using ConsumePrefix, and now we have another would-be
user in cutils. (There appears to be one place in adb that should use
ConsumeSuffix, so I'm assuming we'll want that sooner or later.)

I've kept these inline because adb and google3's versions both were, and
I'm easily led.

Test: treehugger
Change-Id: I29d99032f6f6ccbfaefece59725db8afb02a4c87

This catches a common mistake where client code checks for errors using
the common idiom that works for std::iostream and other file-like
classes:

  unique_fd fd = open(...);
  if (!fd) {
  }

Test: atest libbase_test
Test: m droid
Change-Id: I9629a7795537ecb3b57be9c741c06f80967e4cc2

Test: UTs

Change-Id: I3751656d44e7a814488169c53d3e8af0da44f262

* changes:
  adb: switch to base::{Send,Receive}FileDescriptors.
  debuggerd: switch to base::{Send,Receive}FileDescriptors.
  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

This is a useful helper for anyone that's using thread safety
annotations alongside std::condition_variable, extract it from adb and
move it to libbase.

Test: mma
Change-Id: Ic51e2f2a0e6a16628034b29d8ff32bf2155afccd

unique_fd's destructor potentially mangling errno makes it difficult to
use correctly in code that sets errno (or, in reality, it makes it so
that errno values get randomly stomped upon if close actually sets
errno, because no one accounts for this case).

Preserve errno ourselves to avoid this.

Test: treehugger
Change-Id: Ib06e6f65866d86fff4032b2311021eaf9226a1af

Bug: none
Test: build and use it manually.
Test: run libbase_tests.

Change-Id: I50b5251e8da23ddce94711807859612bf890406b

The use in implicit conversion is intentional.

Test: m
Change-Id: I6c756e772b274e8a525ba2ad792cc4a8e1fc0176