RFR: 8314114: Fix -Wconversion warnings in os code, primarily linux [v2]

[Coleen Phillimore]

> This change fixes various -Wconversion warnings from files in the os directories and related, widening types or adding checked_cast<> where narrowing.
> Tested with tier1 on linux/macosx/windows and tier1-4 on linux-x64-debug, windows-x64-debug.

Coleen Phillimore has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains eight additional commits since the last revision:

 - Dean's suggested changes.
 - Merge branch 'master' into os-conversion
 - Fix cgroupSubsystem_linux.cpp
 - Unset Wconversion for testing.
 - Fix signals to take int buflen, fixed return types of syscalls
 - Fix attachListener.cpp warnings.
 - change send/recv parameter type to match OS.
 - Fix -Wconversion warnings in os_linux.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/15229/files
  - new: https://git.openjdk.org/jdk/pull/15229/files/d289f15b..fd5464d7

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=15229&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=15229&range=00-01

  Stats: 435 lines in 29 files changed: 163 ins; 209 del; 63 mod
  Patch: https://git.openjdk.org/jdk/pull/15229.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15229/head:pull/15229

PR: https://git.openjdk.org/jdk/pull/15229