add TCP_NOTSENT_LOWAT tcp socket option, new in linux v3.12
add O_TMPFILE flag, new in linux 3.11

definition in linux:
 #define O_TMPFILE (__O_TMPFILE | O_DIRECTORY)
where __O_TMPFILE and O_DIRECTORY are arch specific
use 0 instead of NULL in headers where NULL is not defined

some macros in sys/mtio.h and syslog.h used NULL without defining it
math: clean up __rem_pio2

- remove the HAVE_EFFICIENT_IRINT case: fn is an exact integer, so
  it can be converted to int32_t a bit more efficiently than with a
  cast (the rounding mode change can be avoided), but musl does not
  support this case on any arch.
- __rem_pio2: use double_t where possible
- __rem_pio2f: use less assignments to avoid stores on i386
- use unsigned int bit manipulation (and union instead of macros)
- use hexfloat literals instead of named constants