add alpine (musl libc) CI and fix test case with musl

First commit adds alpine with musl libc as test and coverage target to the CI.

Second commit fixes the fpurge testcase with musl libc, alpine currently patches out the test case and void linux removed the static declaration of FILE.

I wasn't able to find a better way of testing passing an invalid FILE, not sure if its possible.

Edit: Added more changes to fpurge, to not use fileno since that breaks fpurge with non fd based FILE implementations like from open_memstream or fopencookie.