There are more than enough calls doing string manipulations to deserve
its own files, hence do something about it.
This patch also sorts the #include blocks of all files that needed to be
updated, according to the sorting suggestions from CODING_STYLE. Since
pretty much every file needs our string manipulation functions this
effectively means that most files have sorted #include blocks now.
Also touches a few unrelated include files.
This moves is_gateway() from nss-myhostname into the basic APIs, and
makes it more like is_localhost(). Also, we rename it to
is_gateway_hostname() to make it more expressive.
Sharing this function in src/basic/ allows us to reuse the function for
routing name requests in resolved (in a later commit).
Tests are modified to check behaviour with relax and without relax.
New tests are added for hostname_cleanup().
Tests are moved a new file (test-hostname-util) because there's
now a bunch of them.
New parameter is not used anywhere, except in tests, so there should
be no observable change.
