MFC r291155:

MFC	r291155:
bc: sync with OpenBSD

tty.c Rev. 1.3
Avoid unintended problems with operator precedence when doing an
assignment and comparison.

bc.1, Rev. 1.31, 1.32
'.Ql Quit' -> '.Ql quit' because only the lowercase command is valid.
Clarify sentence about `quit` in BUGS section.

extern.h, Rev. 1.12
whitespace

bc.y, Rev. 1.47
Prefer setvbuf() to setlinebuf() for portability

Obtained from:	OpenBSD

git-svn-id: svn://svn.freebsd.org/base/stable/10@292753 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f