Allow noise semicolons ending psql \sf, \ef, \sv, \ev commands.

Allow noise semicolons ending psql \sf, \ef, \sv, \ev commands.

Many psql backslash commands tolerate trailing semicolons, even
though that's not part of the official syntax.  These did not.
They tried to, by passing semicolon = true to psql_scan_slash_option,
but that function ignored this parameter in OT_WHOLE_LINE mode.
Teach it to do the right thing, and remove the now-duplicative
logic in exec_command_help.

Discussion: https://postgr.es/m/2012251.1704746912@sss.pgh.pa.us