alsamixer: small optimizations

alsamixer: small optimizations

textbox.c, mixer_display.c:
  - Replaced manual line drawing code `for (...) mvwaddch(...)` by the
    functions `mvwhline/mvwvline` provided by standard curses

card_select.c, proc_files.c:
  - Moved duplicate code in the create() functions to menu_widget_create()
    in menu_widget.c

card_select.c:
  - Removed redundant calls to sprintf()

device_name.c:
  - Combined multiple calls to form_opts_off()
  - Recognize ASCII DEL (127) as backspace

widget.c:
  - Uncommented code for setting a dummy key handler (all existing
    widgets do provide a key handling function)

Signed-off-by: Benjamin Abendroth <braph93@gmx.de>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>