RFR: 8261949: fileStream::readln returns incorrect line string

[Yang Yi]

When the last line does not contain a NEWLINE character, fileStream::readln would read
truncated line string:

$ cat file_content:
AA
BB
CC<EOF>

fileStream::readlnresult:
"AA"
"BB"
"C"

This patch address this problem, it works for Posix and Windows since the last character
of these systems is always '\n'.

-------------

Commit messages:
 - fileStream::readln returns incorrect line string

Changes: https://git.openjdk.java.net/jdk/pull/2626/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2626&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8261949
  Stats: 5 lines in 1 file changed: 3 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2626.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2626/head:pull/2626

PR: https://git.openjdk.java.net/jdk/pull/2626