core: refactor usage() output

Provide usage() by a single function now - not several time in code.

Signed-off-by: Hagen Paul Pfeifer <hagen@jauu.net>
core: add version option

Sometimes it is handy to know what captcp is used. --version will now
print the current version and exit the program.

Signed-off-by: Hagen Paul Pfeifer <hagen@jauu.net>
core: adjust copyright year string

Just to stay in time ...

Signed-off-by: Hagen Paul Pfeifer <hagen@jauu.net>
core: captcp version 0.8

Signed-off-by: Hagen Paul Pfeifer <hagen@jauu.net>