lineedit: preparatory cleanup patch for Unicode completion fix

lineedit: preparatory cleanup patch for Unicode completion fix

Some logic changes and function renames. The fix will follow this patch,
to mkae it distinct from cleanup.

function                                             old     new   delta
build_match_prefix                                     -     892    +892
username_completion                                    -     121    +121
read_line_input                                     4902    4966     +64
username_tab_completion                              235       -    -235
find_match                                           892       -    -892
------------------------------------------------------------------------------
(add/remove: 2/2 grow/shrink: 1/0 up/down: 1077/-1127)        Total: -50 bytes

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