MFC: r289863,r289931,r290110,r290230,r290231,r290232

r290232:

Microoptimize.

r290231:

Addition to prev. commit.
In some edge cases fp->_p can be changed in _sseek(), recalculate.

r290230:

Don't seek to the end if write buffer is empty (in append modes).
PR: 204156

r290110:

Add _flags2 per jhb@ suggestion since no room left in _flags.
Rewrite O_APPEND flag checking using new __S2OAP flag.

r289931:

According to POSIX, a write operation shall start at the current size of
the stream (if mode had 'a' as the first character).

r289863:

Since no room left in the _flags, reuse __SALC for O_APPEND.
It helps to remove _fcntl() call from _ftello() and optimize seek position
calculation in _swrite().