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
MFC after: 2 weeks