Rename TARGET_XFER_E_UNAVAILABLE to TARGET_XFER_UNAVAILABLE

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.