MFC r306568, r306569

Add the __printflike attribute to the declarations of
dprintf(3) and vdprintf(3).

Sponsored by: Dell EMC

MFC r285140:
r285140 (by oshogbo):

Add fdclose(3) function.

This function is equivalent to fclose(3) function except that it
does not close the underlying file descriptor.
fdclose(3) is step forward to make FILE structure private.

Reviewed by: wblock, jilles, jhb, pjd
Approved by: pjd (mentor)
Differential Revision: https://reviews.freebsd.org/D2697

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().

MFC r278600:

Correct and clarify comment for __SMBF.

MFC r278600:

Correct and clarify comment for __SMBF.