Improve acl file parsing robustness

We now support comments, empty lines, and leading whitespace (which all get
skipped)

Lines with junk will _probably_ get skipped too, depending on how bad
the junk really is.
Bring documentation in sync with code
Separate the test suite out into a separate directory

... in preparation of also adding a "code" test suite which tests
functions in the code, not runtime behaviour of nbd-server
Make a start with a library

The library does not contain much useful stuff yet, so make it a
noinst_LTLIBRARY for now, rather than a lib_LTLIBRARY.

As we move code to the library, we'll modify it so that it becomes
selfsufficient, and clean it up as we go.

For now, only move the freshly rewritten authorized_client() function
there.

This adds libtool to the list of stuff needed when compiling from git,
but that's not much of an issue.
Add a "code" test suite

This test suite will test functions in libnbdsrv.la. For now, only test
that the authorized_client function does what it's supposed to do.