Remove unnecessary includes from mxsh.c

We included stdio.h so that we could use fgets when implementing the
command line prompt in mxsh. We have since then moved the implementation
of the prompt to separate files and thus no longer need to include it in
mxsh.c. Remove it since it is unnecessary, and also remove the inclusion
of glib-object.h which we also don't need.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>
Parse command line into array of arguments

The command line is not parsed in any particular way. We just check if
it is equal to some string. This works for simple commands which doesn't
need any arguments. But some commands will need arguments. It should be
possible to specify the name of the user when logging in, or the name of
the mailbox to select. We need to parse the command line before we can
act on it.

Implement command line parsing using a state machine which parses the
line into an array of arguments. Only look at the first argument when
checking for the command name.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>
Show an error when the command is not known

Nothing will happen if the user invokes an unknown command. The prompt
will just return. Show an error message if that happens so that the user
knows that nothing has happened.

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