Remove unnecessary command prototypes

The command prototypes are defined in columbia-cli.h, but it's not
necessary for them to be there. The only function that needs to know
about them is find_command, which is in the same file. Remove the
prototypes from the header so that they don't have to be maintained
there.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>
Remove C++ extern "C" from private header

The public header contains a conditional extern "C" { ... } in order to
build with a C++ compiler. This is also present in the private header.
It's not necessary to keep it in the private header since we only expect
it to be built with a C compiler, and it can thus be removed.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>