Fix typo in documentation and fix issues on kvs_fs

It started with a simple documentation review, and ended with some
fixes on kvs_fs:

  - use filename:join instead of concatenation (crafted key can
    write data anywhere on the system)

  - add kvs_fs:delete/2 supports

vsn

7.1.1

11 dialyzer errors left

remove active

OTP 17 compliance

travis

make 3.9 better again