C++ify fileio_fh_t, replace VEC with std::vector

C++ify fileio_fh_t, replace VEC with std::vector

Preparation for the next patch.

- Replace VEC with std::vector.
- Rewrite a couple macros as methods/functions.
- While at it, rename fileio_fh_t::fd as fileio_fh_t::target_fd to
  avoid confusion between target and host file descriptors.

gdb/ChangeLog:
2018-04-11  Pedro Alves  <palves@redhat.com>

	* target.c (fileio_fh_t): Make it a named struct instead of a
	typedef.
	(fileio_fh_t::is_closed): New method.
	(DEF_VEC_O (fileio_fh_t)): Remove.
	(fileio_fhandles): Now a std::vector.
	(is_closed_fileio_fh): Delete.
	(acquire_fileio_fd): Adjust.  Rename parameters.
	(release_fileio_fd): Adjust.
	(fileio_fd_to_fh): Reimplement as a function instead of a macro.
	(target_fileio_pwrite, target_fileio_pread, target_fileio_fstat)
	(target_fileio_close): Adjust.