Fix offset caching for streams and use it for ftell (BZ #16680)

The ftell implementation was made conservative to ensure that
incorrectly cached offsets never affect it.  However, this causes
problems for append mode when a file stream is rewound.  Additionally,
the 'clever' trick of using stat to get position for append mode files
caused more problems than it solved and broke old behavior.  I have
described the various problems that it caused and then finally the
solution.

For a and a+ mode files, rewinding the stream should result in ftell
returning 0 as the offset, but the stat() trick caused it to
(incorrectly) always return the end of file.  Now I couldn't find
anything in POSIX that specifies the stream position after rewind()
for a file opened in 'a' mode, but for 'a+' mode it should be set to
0.  For 'a' mode too, it probably makes sense to keep it set to 0 in
the interest of retaining old behavior.

The initial file position for append mode files is implementation
defined, so the implementation could either retain the current file
position or move the position to the end of file.  The earlier ftell
implementation would move the offset to end of file for append-only
mode, but retain the old offset for a+ mode.  It would also cache the
offset (this detail is important).  My patch broke this and would set
the initial position to end of file for both append modes, thus
breaking old behavior.  I was ignorant enough to write an incorrect
test case for it too.

The Change:

I have now brought back the behavior of seeking to end of file for
append-only streams, but with a slight difference.  I don't cache the
offset though, since we would want ftell to query the current file
position through lseek while the stream is not active.  Since the
offset is moved to the end of file, we can rely on the file position
reported by lseek and we don't need to resort to the stat() nonsense.

Finally, the cache is always reliable, except when there are unflished
writes in an append mode stream (i.e. both a and a+).  In the latter
case, it is safe to just do an lseek to SEEK_END.  The value can be
safely cached too, since the file handle is already active at this
point.  Incidentally, this is the only state change we affect in the
file handle (apart from taking locks of course).

I have also updated the test case to correct my impression of the
initial file position for a+ streams to the initial behavior.  I have
verified that this does not break any existing tests in the testsuite
and also passes with the new tests.
Change offset in fdopen only if setting O_APPEND

fdopen should only be allowed to change the offset in the file it
attaches to if it is setting O_APPEND.  If O_APPEND is already set, it
should not change the state of the handle.
Fix __PTHREAD_MUTEX_HAVE_ELISION -Wundef warning

ChangeLog:

2014-03-17  Will Newton  <will.newton@linaro.org>

	* nptl/sysdeps/pthread/pthread.h: Check
	__PTHREAD_MUTEX_HAVE_ELISION is defined before testing
	its value.
Fix HP_SMALL_TIMING_AVAIL -Wundef warnings

ChangeLog:

2014-03-17  Will Newton  <will.newton@linaro.org>

	* sysdeps/generic/ldsodefs.h: Check whether
	HP_SMALL_TIMING_AVAIL is defined with #ifdef rather
	than #if.
Fix _IO_JUMPS_OFFSET -Wundef warnings

ChangeLog:

2014-03-17  Will Newton  <will.newton@linaro.org>

	* libio/genops.c: Check whether
	_IO_JUMPS_OFFSET is defined with #ifdef rather
	than #if.
	* libio/libioP.h: Likewise.
	* stdio-common/vfprintf.c: Likewise.
Fix __STRICT_ANSI__ -Wundef warnings

ChangeLog:

2014-03-17  Will Newton  <will.newton@linaro.org>

	* argp/argp-fmtstream.h: Check whether
	__STRICT_ANSI__ is defined with #ifdef rather
	than #if.
	* argp/argp.h: Likewise.
Fix HAVE_RM_CTX -Wundef warnings

ChangeLog:

2014-03-17  Will Newton  <will.newton@linaro.org>

	* sysdeps/generic/math_private.h: Check whether
	HAVE_RM_CTX is defined with #ifdef rather
	than #if.