Modernize finduser() a bit

Skip any prepended white-space in the passed name to find.
Use better variable names (and length) for the character strings.
Use matchusername() instead of strcmp(); matchusername() follows the liberal user-id/alias/name matching algorithm.
Use strcasestr() instead of strstr(), so no copy/modification (upper-casing) of passed name is now required.
When 'Q'uitting the search, set the SS_ABORT flag.