Unbreak mips native build

When I build mips native gdb today, I get the follow error,

../../../git/gdb/mips-linux-nat.c: In function '_initialize_mips_linux_nat':
../../../git/gdb/mips-linux-nat.c:792:15: error: assignment from incompatible pointer type [-Werror]
cc1: all warnings being treated as errors

It looks an oversight of recent target_ops delegation patches.  This
patch is to fix the build error.

gdb:

2014-02-24  Yao Qi  <yao@codesourcery.com>

	* mips-linux-nat.c (super_close): Update its type.
	(mips_linux_close): Pass 'self' to super_close.
Tweak target_xfer_status_to_string

This patch tweaks target_xfer_status_to_string on comments and argument
name.

gdb:

2014-02-24  Yao Qi  <yao@codesourcery.com>

	* target.c (target_xfer_status_to_string): Rename argument err
	to status.
	* target.h (target_xfer_status_to_string): Update declaration.
	Replace target_xfer_error_to_string with
	target_xfer_status_to_string in comment.
Rename TARGET_XFER_E_UNAVAILABLE to TARGET_XFER_UNAVAILABLE

Nowadays, TARGET_XFER_E_UNAVAILABLE isn't regarded as an error in
to_xfer_partial interface, so _E_ looks odd.  This patch is to
replace TARGET_XFER_E_UNAVAILABLE with TARGET_XFER_UNAVAILABLE,
and change its value from -2 to 2.  Since there is no comparison
on the value of 'enum target_xfer_status', so it should be safe.

gdb:

2014-02-24  Yao Qi  <yao@codesourcery.com>

	* target.h (enum target_xfer_status)
	<TARGET_XFER_E_UNAVAILABLE>: Rename it to ...
	<TARGET_XFER_UNAVAILABLE>: ... it with setting value 2
	explicitly.  New.
	* corefile.c (memory_error_message): User updated.
	* exec.c (section_table_read_available_memory): Likewise.
	* record-btrace.c (record_btrace_xfer_partial): Likewise.
	* target.c (target_xfer_status_to_string): Likewise.
	(raw_memory_xfer_partial): Likewise.
	(memory_xfer_partial_1, target_xfer_partial): Likewise.
	* valops.c (read_value_memory): Likewise.
	* exec.h: Update comments.
Fix a GDB assert failure on windows

A GDB internal error is found on native mingw32 target.

(gdb) run
../../binutils-gdb/gdb/target.c:1483: internal-error:
target_xfer_partial: Assertion `*xfered_len > 0' failed.
A problem internal to GDB has been detected,
further debugging may prove unreliable.
Quit this debugging session? (y or n)

This error was introduced by the following snippet in commit
9b409511d07fe375284701af34909fb539029caf

> @@ -2536,27 +2538,30 @@ windows_xfer_shared_libraries (struct target_ops *ops,
>      }
>
>    obstack_free (&obstack, NULL);
> -  return len;
> +  *xfered_len = (ULONGEST) len;
> +  return TARGET_XFER_OK;
>  }

In the original code, len is returned, which could be 0, but after that
commit, only TARGET_XFER_OK is returned, which is wrong.  If len is 0,
TARGET_XFER_EOF should be returned.  (it is 0 in enum
target_xfer_status declaration).

gdb:

2014-02-24  Yuanhui Zhang  <asmwarrior@gmail.com>

	* windows-nat.c (windows_xfer_shared_libraries): Return
	TARGET_XFER_EOF if LEN is zero to fix an assert failure when
	requested object is TARGET_OBJECT_LIBRARIES.