client: add --maximized and --fullscreen

We now create a copy of the config for each client, and updates it
with the values passed from the client.

Since we're not actually cloning it (and e.g. strdup() all strings
etc) we can't call conf_destroy() to free it, but need to free just
the strings we've replaced.

completions/zsh/_footclient

@@ -3,6 +3,8 @@
 _arguments \
     -s \
     '(-t --term)'{-t,--term}'[value to set the environment variable TERM to (foot)]:term:->terms' \
+    '--maximized[start in maximized mode]' \
+    '--fullscreen[start in fullscreen mode]' \
     '--login-shell[start shell as a login shell]' \
     '(-s --server-socket)'{-s,--server-socket}'[override the default path to the foot server socket (XDG_RUNTIME_DIR/foot.sock)]:server:_files' \
     '(-l --log-colorize)'{-l,--log-colorize}'[enable or disable colorization of log output on stderr]:logcolor:(never always auto)' \