Add unique_fd::operator{==,!=} overloads that take a unique_fd.

With C++20, any use of the existing overloads with two unique_fd
arguments is likely to become ambiguous:
http://lists.llvm.org/pipermail/cfe-commits/Week-of-Mon-20200113/301880.html

Newer versions of clang complain about this ambiguity. Fix the error by
adding overloads that take two unique_fds.

Bug: 145916209
Change-Id: I18a292827d8841b6d24f948682123ab54dc7aaca