Replace _sendflags_str() and _recvflags_str() with _msg_flags_str().

Replace _sendflags_str() and _recvflags_str() with _msg_flags_str().

* tapset/linux/aux_syscalls.stp (_msg_flags_str): New function to replace
  _sendflags_str() and _recvflags_str().
  (_sendflags_str): Deprecated and reimplemented with _msg_flags_str().
  (_recvflags_str): Ditto.
* tapset/linux/syscalls2.stp: Replace _sendflags_str() and
  _recvflags_str() with _msg_flags_str().
* tapset/linux/nd_syscalls2.stp: Ditto.
* testsuite/buildok/aux_syscalls-embedded.stp: Test _msg_flags_str().
* NEWS: Mention deprecations.