getdelim(3): Fix losing data on [EAGAIN]

Currently when an [EAGAIN] is encountered we return a partial result
that does not contain the delimeter. On the next (successful) read we
were returning the next part of the line without the preceding string
from the first failed call.

Fix this by using the same mechanism as ungetc(3) does. For the buffered
case we could simply set fp->_r and fp->_p back to their values before
sappend() is ran but for simplicity ungetc(3) is done in there as well.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D31687