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

[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.

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

Commit messages:
 - 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.

Changes: https://git.openjdk.org/jdk/pull/15229/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15229&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8314114
  Stats: 125 lines in 21 files changed: 0 ins; 0 del; 125 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