Add support for zOS

[v1gnesh]

Hi,

Thank you for building cool stuff!

zOS is an IBM Z (mainframe) operating system.
This PR adds the appropriate builds tags to support it.

Tests pass and look like this:

go: downloading github.com/google/go-cmp v0.3.0
?   	github.com/apparentlymart/go-userdirs/internal/unix	[no test files]
?   	github.com/apparentlymart/go-userdirs/macosbase	[no test files]
=== RUN   TestDirsNewPath
=== RUN   TestDirsNewPath/NewConfigPath
=== RUN   TestDirsNewPath/NewDataPath
=== RUN   TestDirsNewPath/CachePath
--- PASS: TestDirsNewPath (0.00s)
    --- PASS: TestDirsNewPath/NewConfigPath (0.00s)
    --- PASS: TestDirsNewPath/NewDataPath (0.00s)
    --- PASS: TestDirsNewPath/CachePath (0.00s)
=== RUN   TestDirsFindConfigFiles
=== RUN   TestDirsFindConfigFiles/both
=== RUN   TestDirsFindConfigFiles/only_one
=== RUN   TestDirsFindConfigFiles/none
--- PASS: TestDirsFindConfigFiles (0.00s)
    --- PASS: TestDirsFindConfigFiles/both (0.00s)
    --- PASS: TestDirsFindConfigFiles/only_one (0.00s)
    --- PASS: TestDirsFindConfigFiles/none (0.00s)
=== RUN   TestDirsFindDataFiles
=== RUN   TestDirsFindDataFiles/both
=== RUN   TestDirsFindDataFiles/none
--- PASS: TestDirsFindDataFiles (0.00s)
    --- PASS: TestDirsFindDataFiles/both (0.00s)
    --- PASS: TestDirsFindDataFiles/none (0.00s)
=== RUN   TestDirsGlobConfigFiles
=== RUN   TestDirsGlobConfigFiles/all
=== RUN   TestDirsGlobConfigFiles/none
--- PASS: TestDirsGlobConfigFiles (0.00s)
    --- PASS: TestDirsGlobConfigFiles/all (0.00s)
    --- PASS: TestDirsGlobConfigFiles/none (0.00s)
=== RUN   TestDirsGlobDataFiles
=== RUN   TestDirsGlobDataFiles/all
=== RUN   TestDirsGlobDataFiles/none
--- PASS: TestDirsGlobDataFiles (0.00s)
    --- PASS: TestDirsGlobDataFiles/all (0.00s)
    --- PASS: TestDirsGlobDataFiles/none (0.00s)
PASS
ok  	github.com/apparentlymart/go-userdirs/userdirs	0.442s
?   	github.com/apparentlymart/go-userdirs/windowsbase	[no test files]
?   	github.com/apparentlymart/go-userdirs/xdgbase	[no test files]

[v1gnesh]

Hi @apparentlymart, when you have a chance, could you please review & pull this in?

[v1gnesh]

Also pinging @ajacoutot...

[ajacoutot]

Also pinging @ajacoutot...

Me?
What do I have to do with zOS? :-)

[v1gnesh]

Hi!
Not with zOS but I noticed only yourself and apparentlymart as the contributors.
And now I think... does contributor mean the same as maintainer 😛 , i.e., will you be able to accept the PR.