os/linux.rb: don't show codename when it's n/a

[maxim-belkin]

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Some Linux distros don't have codenames, so lsb_release -c returns n/a. brew config shows the OS that Homebrew detects and adds OS codename in parentheses. It can be somewhat confusing when codename is equal to n/a:

OS: SUSE Linux Enterprise Server 11 (x86_64) (n/a)

Here I'm changing the output of os_version function so that we don't show codename when it's n/a. Also, instead of using outputs of lsb_release -d and lsb_release -c separately, I'm calling lsb_release with -a flag to get all the information in one step and then parse the output to obtain the required information.

[BrewTestBot]

Review period will end on 2020-12-01 at 20:51:04 UTC.

[MikeMcQuaid]

Fine with me but will need rebased before CI is 🟢

[maxim-belkin]

rebased

[BrewTestBot]

Review period ended.

[MikeMcQuaid]

Thanks @maxim-belkin!

[sjackman]

Thanks, Maxim!