libbb: introduce xmalloc_xopen_read_close and use where appropriate

libbb: introduce xmalloc_xopen_read_close and use where appropriate
instead of xmalloc_open_read_close.

function                                             old     new   delta
xmalloc_xopen_read_close                               -      34     +34
xmalloc_open_read_close                              163     171      +8
passwd_main                                         1070    1074      +4
rexecve                                              254     257      +3
handle_incoming_and_exit                            2657    2659      +2
parse_command                                       1509    1510      +1
buffer_fill_and_print                                 76      73      -3
evaltreenr                                           599     589     -10
evaltree                                             599     589     -10
------------------------------------------------------------------------------
(add/remove: 1/0 grow/shrink: 5/3 up/down: 52/-23)             Total: 29 bytes