system_config: add umask option to <system> directive

[somaz94]

Which issue(s) this PR fixes: Fixes #4816

What this PR does / why we need it:
Adds a umask option to the <system> directive so the process umask can be
set from the config file. Previously this required the --umask command line
option, which is inconvenient for services and container images.

It reuses the existing chumask handling: <system> umask sets the same value
that --umask does, and the command line option takes precedence when both are
given. Both supervised and --no-supervisor modes honor it.

<system>
  umask 0022
</system>

Tests: added a config round-trip case and a supervisor test asserting the value
reaches ServerEngine, including command-line precedence.

Docs Changes:
Follow-up in fluent/fluentd-docs-gitbook (the <system> directive docs).

Release Note:
Add umask option to the <system> directive.