RFR: 8183227: read/write APIs in class os shall return ssize_t

[Harold Seigel]

Please review this small fix that changes the return type of os::write() to ssize_t.  No changes were needed for os::read() because its return type is ssize_t.  This fix was tested by running Mach5 tiers 1-2 on Linux, Mac OS, and Windows and Mach5 tiers 3-5 on Linux x64.
Thanks, Harold

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Commit messages:
 - 8183227: read/write APIs in class os shall return ssize_t

Changes: https://git.openjdk.java.net/jdk/pull/6992/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6992&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8183227
  Stats: 39 lines in 10 files changed: 3 ins; 4 del; 32 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6992.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6992/head:pull/6992

PR: https://git.openjdk.java.net/jdk/pull/6992