Fix a bug introduced by r251236; product of bug was the following error in
"Startup"->"View/Edit Startup Configuration"->"Add New"->"Add From List":

[: -eq: unexpected operator

NOTE: Previously mentioned in r251557

Problem caused by removal of $retval to capture exit status of call to
f_dialog_input_rclist().

While we're here, enhance f_dialog_input_rclist() to accept a default-item
to make selecting multiple values to-add more efficient.