cli: search: Convert ctx. to ctx->

In the next commit, notmuch_search_command will be refactored to
several smaller functions. In order to simplify the next commit to
verbatim move of several lines to new functions with search_context_t*
argument, we convert all references to this structure to pointer
dereferences. To do so we rename the context variable and use the
original name ctx as the pointer to the renamed structure.