What does this PR do?

Unify result processing for timed_subprocess.TimedProc and win_runas.runas

• use the same code path for both functions
• fix stdout/stderr decoding exceptions with runas as previously UTF-8 was assumed

win_runas.runas and win_runas.runas_unpriv

• support the parameter env to pass custom environment variables
• support the parameter redirect_stderr to redirect stderr to stdout
• support the parameter timeout to abort execution and kill the process after a timeout
• support the parameter bg to run a command in background without waiting for the result
• ensure opened handles are closed

Merge requirements satisfied?

• Docs
• Changelog - https://docs.saltproject.io/en/master/topics/development/changelog.html
• Tests written/updated

Commits signed with GPG?

No