Split buffer.h into smaller units.

Split the header file `buffer.h` into smaller headers.
Split network.h in smaller units

As reported in issue #37, header `network.h` was split in several header
files.
Split stream.h into smaller units

As reported in issue #37, `stream.h` is splitted in smaller units.
Split fsuipc.h into smaller units

As reported in issue #37, `liboac/fsuipc.h` is splitted into smaller
units.
Split api.h into smaller units

As reported in issue #37, `flightvars/api.h` is splitted into smaller
units.
Split protocol.h into smaller units

As reported in issue #37, `flightvars/protocol.h` is splitted into smaller
units.
Fix include directives

Use system header include style for libraries and local include style for
headers that belong to the executables.
Rename liboac-commons to simply liboac

The commons suffix is meaningless and should be removed.
Split unit tests corresponding to split headers

As issue #37 is addressed, some headers were split. This change splits
their unit tests as well with the exception of buffer, which depends on a
DSL shared by all tests.
Split subscriptions.h and generalize errors

As reported in issue #30, the errors of `subscription_mapper` had to be
generalized. That's done in this change by splitting `subscriptions.h`
into smaller units and adding the corresponding new abstract error in
`subscription/errors.h`.