Introduce 'rabbitmq-diagnostics erlang_cookie_sources'

[michaelklishin]

to help troubleshoot authentication issues.

I could not quickly locate an Erlang function that would return the expected path in OTP so
we reinvent what this BIF does in C to cover the Windows-specific special handling of the HOMEDRIVE and HOMEPATH env variables.

Example output:

Cookie File

Effective user: user
Effective home directory: /Users/user
Cookie file path: /Users/user/.erlang.cookie
Cookie file exists? true
Cookie file type: regular
Cookie file access: read
Cookie file size: 20

Cookie CLI Switch

--erlang-cookie value set? false
--erlang-cookie value length: 0

Env variable  (Deprecated)

RABBITMQ_ERLANG_COOKIE value set? false
RABBITMQ_ERLANG_COOKIE value length: 0

Inspired by an idea from @gerhard.

[michaelklishin]

Backported to v3.8.x.