Corrected numerous minor regressions.

The grep_path_recursively_caselessly*() family of functions now defaults
unpassed arguments as documented. is_alias() now behaves silently, as expected.
print_alias() now throws exceptions when at least one such alias is undeclared.
For effiency, former functions is_shell_interactive() and is_shell_login() are
now aliases. (And doubtlessly witlessly more.)