MFC r304607,r304641,r304819,r304811

1. Don't forget to set SERR on slbexpand() error.

2. Remove "Fast path" from fgetwc()/fputwc() since it can't detect

encoding errors and ignores them all.
One of affected encoding example: US-ASCII

3. Original fgetln() from 44lite return success for line tail errors,

i.e. partial line, but set __SERR and errno in the same time, which
is inconsistent.
Now both OpenBSD and NetBSD return failure, i.e. no line and set error
indicators for such case, so make our fgetln() and fgetwln()
(as its wide version) compatible with the rest of *BSD.

PR: 212033