Eliminate target_fileio_open_warn_if_slow

Eliminate target_fileio_open_warn_if_slow

This basically makes target_fileio_open_1 extern, renamed to
target_fileio_open, and eliminates the current
target_fileio_open_warn_if_slow and target_fileio_open.

A following parameter will want to change gdb_bfd_iovec_fileio_open,
the only caller of target_fileio_open_warn_if_slow, to pass down
"warn_if_slow" true/false from the caller, instead of hardcoding
"warn_if_slow" true.

gdb/ChangeLog:
2020-05-19  Pedro Alves  <palves@redhat.com>

	* gdb_bfd.c (gdb_bfd_iovec_fileio_open): Adjust.
	* target.c (target_fileio_open_1): Rename to target_fileio_open
	and make extern.  Use bool.
	(target_fileio_open, target_fileio_open_warn_if_slow): Delete.
	(target_fileio_read_alloc_1): Adjust.
	* target.h (target_fileio_open): Add 'warn_if_slow' parameter.
	(target_fileio_open_warn_if_slow): Delete declaration.