* descriptors.cc: New file.
* descriptors.h: New file.
* gold-threads.h (class Hold_optional_lock): New class.
* fileread.cc: Include "descriptors.h".
(File_read::~File_read): Release descriptor rather than closing
it.
(File_read::open) [file]: Call open_descriptor rather than open.
Set is_descriptor_opened_.
(File_read::open) [memory]: Assert that descriptor is not open.
(File_read::reopen_descriptor): New function.
(File_read::release): Release descriptor.
(File_read::do_read): Make non-const. Reopen descriptor.
(File_read::read): Make non-const.
(File_read::make_view): Reopen descriptor.
(File_read::do_readv): Likewise.
* fileread.h (class File_read): Add is_descriptor_opened_ field.
Update declarations.
* layout.cc: Include "descriptors.h".
(Layout::create_build_id): Use open_descriptor rather than open.
* output.cc: Include "descriptors.h".
(Output_file::open): Use open_descriptor rather than open.
* archive.cc (Archive::const_iterator): Change Archive to be
non-const.
(Archive::begin, Archive::end): Make non-const.
(Archive::count_members): Likewise.
* archive.h (class Archive): Update declarations.
* object.h (Object::read): Make non-const.
* Makefile.am (CCFILES): Add descriptors.cc.
(HFILES): Add descriptors.h.
* Makefile.in: Rebuild.
