mutt: handle neomutt -Q var correctly

[mgedmin]

The $muttcmd -Q ... | sed ... invocation was repeated in three places, so I've extracted it into a new helper function _muttconfvar().

Closes #462.

(I looked into adding a test, but I'm not sure how. It looks like the tests are executed in Docker, but I couldn't figure out what packages are included there or how to add neomutt.)

[mgedmin]

Packages get included in docker images automatically, the image builds reference test/test-cmd-list.txt to determine what gets pulled in,

neomutt in debian buster (10) is old enough that it still prints using the old output format.

neomutt in ubuntu 20.04 is using the new format, but it looks like your .travis.yml runs against ubuntu14. (neomutt in ubuntu 18.04 also uses the old format.)

I don't know how to check package names and versions available for centos, fedora or alpine. The name 'fedoradev' seems promising if one wants fresh new package versions.

Anyway it looks like testing this properly would need #466 to be merged as well.

[mgedmin]

So I have some tests in #467. In my environment 'mutt' is an alias for 'neomutt', so the tests I added fail without this fix.

I could copy the _muttconfvar test from test_mutt.py in #467 into test_neomutt.py added in #466, and then the fix would be tested in CI.

(I don't see any Travis builds here, but I do get them on my fork: https://travis-ci.com/github/mgedmin/bash-completion/branches. The failures I see there are in TestPkgConfig::test_variable and TestAvahiBrowse::test_service_types and I think I saw one failure about dselect, but I can't find it at the moment.)