add parameter use_answer to search_init(); when it's TRUE, only set

add parameter use_answer to search_init(); when it's TRUE, only set
backupstring to answer, so that we can preserve the text of the
statusbar when switching to the search prompt from the "Go To Line"
prompt; also, set backupstring before doing anything else, add one minor
efficiency tweak, and preserve the text of the statusbar no matter what
when switching from the search prompt to the "Go To Line" prompt, since
the toggling works both ways now and non-numeric text shouldn't be lost
when going only one of those ways

git-svn-id: svn://svn.savannah.gnu.org/nano/trunk/nano@1965 35c25a1d-7b9e-4130-9fde-d3aeb78583b8