less: update line input so that it doesn't interfere with

less: update line input so that it doesn't interfere with
screen update. Makes "man bash", [enter], [/],
<enter search pattern>, [enter] more usable - manpage
draws as you enter the pattern! Yay!!
less: fix bug where backspace wasn't actually deleting chars
less: "examine file with empty name" doesn't abort anymore.
libbb: add "all fatal signals" mask.

getch_nowait                                           -     207    +207
status_print                                           -     105    +105
examine_file                                          64      87     +23
move_cursor                                            -      16     +16
m_status_print                                       185     195     +10
less_main                                           1656    1663      +7
decode_format_string                                 790     795      +5
test_main                                            403     405      +2
process0_stdin                                       247     249      +2
passwd_main                                         1070    1072      +2
less_gets                                            196     178     -18
buffer_print                                         169      71     -98
less_getch                                           362     138    -224
------------------------------------------------------------------------------
(add/remove: 3/0 grow/shrink: 7/3 up/down: 379/-340)           Total: 39 bytes
   text    data     bss     dec     hex filename
 798329     740    7484  806553   c4e99 busybox_old
 798368     740    7484  806592   c4ec0 busybox_unstripped