This extends the "override semantics" section. Also, it rewords or
fixes other parts of the documentation.

Some of the configured values can be overridden with command line
flags, so it would be nice to mention which ones for each kind.

build: Use a better wording in configure.ac (conflicting -> mutually
exclusive), reformat ldflags for clarity in rkt.mk

config: Document some public structs, remove a useless call to
os.TempDir(), make sure that data directory is an absolute path, fix a
bug in toArray

functional tests: Minor code shuffling and renaming in test aci
server.

image: print a message when using stage1 image from a hash

This setting will be later used by --stage1-from-dir flag.

It replaces the --stage1-image flag with five mutually-exclusive
flags: --stage1-url, --stage1-path, --stage1-name, --stage1-hash and
--stage1-from-dir. The flags tell rkt to use stage1 image,
respectively, from a URL, from a path, in the store under the name, in
the store with the hash, from a file in the default stage1 images
directory.

In case none of the above flags were provided, rkt takes stage1 and
paths configuration and build-time defaults to decide for which name
and version of the stage1 image it should search for in the image
store and, if that fails, from which location it should fetch the
image.

This introduces a new configure flag for defining the path to the
default stage1 images directory.

…nder http

This will be used in the stage1 loading test to test stage1 loading
either from a configuration with an http location defined or from the
--stage1-url flag with an http url.

This will be used in the stage1 loading test to make sure that rkt
reuses a stage1 image from the store instead of fetching it from the
remote location again.

…truct

The list of options will grow, which will make the NewServer function
even more awkward (with 5 parameters or so). So this commit puts all
the configuration into the ServerSetup struct and provides a function
for getting a default setup, which can be modified before passing it
to NewServer function.

…ndom one

Choosing a fixed port will make the server to listen on port 80 or
443, depending on whether it uses http or https protocol.

This also replaces a hack for running a discovery server with a proper
solution, which is replacing a default listener with our own.

This implementation is actually so minimal, that it does not even
bother with running stage2.

We can use it regardless of how the project was configured. This
stage1 image could also be used in other tests that do not involve
checking things happening inside stage1 or stage2.

This will be used in stage1 loading test.

The stage1 loading test will use stub stage1 image for its checks. One
of them is testing whether rkt falls back to fetching stage1 image
from the same directory the rkt binary is in. To do this, we need to
put the stub stage1 image in the same directory as rkt. To avoid a
confusion from packagers and users, we do it backwards - we put a
symlink to the rkt binary in the directory where stub stage1 image is.

In the same directory we also put symlinks to all the flavors of the
stage1 image, so the tests should work as they did so far.

It tests loading stage1 from the configuration and from the
flags. Docker cases are not tested at all - we need a docker test
server first.