In a later commit we'll add a new generic client function used by
nmcli and nmtui. nm-client-utils.c seems the right place for it, so
move the file to the base library that is used by both clients.
While at it, also put in that file some functions that will be needed
by nmtui.
These functions are only used by nm-meta-setting-desc.c. Make them internal.
Unfortunately, they are part of "common.h" which cannot be used without
the rest of nmcli. Still todo.
