preparatory cleanup patch for tab completion fixes

preparatory cleanup patch for tab completion fixes

renames and deinlines a few functions

function                                             old     new   delta
input_tab                                              -    1041   +1041
complete_cmd_dir_file                                  -     699    +699
complete_username                                      -     121    +121
username_completion                                  121       -    -121
read_line_input                                     5002    3313   -1689
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(add/remove: 3/1 grow/shrink: 0/1 up/down: 1861/-1810)         Total: 51 bytes

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>