Not sure why the code didn't use this function previously, but
getline(3) is mandatory since POSIX.1-2008 and should thus be supported
by all maintstream operating systems. The proposed changes make the code
easier to read and should also make it more efficient memory-wise as it
does not necessarily cause the entire input to be copied into memory at
once.
and provide BEMENU_BACKEND env variable. Change API to reflect this
change.
There is no reliable way to detect when running on terminal (especially
when stdin is a pipe), so we need to make curses backend explicit with
BEMENU_BACKEND=curses or bm_menu_new("curses"), otherwise GUI backend
will be choosed automatically.
