ftell: seek to end only when there are unflushed bytes (BZ #17647)

Currently we seek to end of file if there are unflushed writes or the
stream is in write mode, to get the current offset for writing in
append mode, which is the end of file.  The latter case (i.e. stream
is in write mode, but no unflushed writes) is unnecessary since it
will only happen when the stream has just been flushed, in which case
the recorded offset ought to be reliable.

Removing that case lets ftell give the correct offset when it follows
an ftruncate.  The latter truncates the file, but does not change the
file position, due to which it is permissible to call ftell without an
intervening fseek call.

Tested on x86_64 to verify that the added test case fails without the
patch and succeeds with it, and that there are no additional
regressions due to it.

	[BZ #17647]
	* libio/fileops.c (do_ftell): Seek only when there are
	unflushed writes.
	* libio/wfileops.c (do_ftell_wide): Likewise.
	* libio/tst-ftell-active-handler.c (do_ftruncate_test): New
	test case.
	(do_one_test): Call it.
tst-ftell-active-handler: Open file with O_TRUNC for w modes

The test case fails to truncate the file when a file is intended to be
opened in w or w+ mode.  Add O_TRUNC to fix this.  The test still
succeeds with this change.
Reset cached offset when reading to end of stream (BZ #17653)

POSIX allows applications to switch file handles when a read results
in an end of file.  Unset the cached offset at this point so that it
is queried again.
Fix up function definition style

Don't use K&R style for function definitions.
Fix date in ChangeLog
Fix another typo in the ChangeLog
Fix linknamespace h_errno handling.

Some linknamespace tests were failing because of references to
h_errno.  References to this symbol should be allowed for XPG4, UNIX98
and XOPEN2K; this patch adds it to the list of allowed data symbols
for those standards.  (h_errno was removed in the 2008 edition of
POSIX, so POSIX2008 and XOPEN2K8 references to it are genuine bugs;
fortunately it's not a public data symbol - __h_errno_location is
public, TLS h_errno is GLIBC_PRIVATE - so the symbol can simply be
renamed to __h_errno to fix those bugs without the ABI complications
normally associated with namespace issues for data symbols.)

Tested for x86_64.

	* conform/list-header-symbols.pl (%extra_syms): Add h_errno for
	XPG4, UNIX98 and XOPEN2K.
	* conform/Makefile (test-xfail-XOPEN2K/grp.h/linknamespace):
	Remove variable.
	(test-xfail-XOPEN2K/pwd.h/linknamespace): Likewise.
De-warning a few stubs.