Add a new static function variable_id::parse()

This function may be used to parse an string and convert it into a
`variable_id` object.
Fix a relative use of format() in exception.h

The macro OAC_MAKE_EXCEPTION was invoking `format()` function without
using its canonical name. This causes a compilation error when used from
another namespace than `oac`.
Fix FV server unawareness of connection reset

As reported in issue #44, `flight_vars_server` was unaware when client
closes or resets its connection. This is fixed in this commit.
Fix a memory access error in local FSUIPC

As reported in issue #45, there was a memory access error in
`local_user_adapter` class, fixed in this commit.
Implement FlightVars Explorer

As reported in issue #4, a new executable program FlightVars Explorer is
added. This program may be used to watch FV variables and set their
values.