Rewrite command-line parser

Now supports optional args followed by non-optional.
New "str.h" functions: strn_str_cmp(), strn_str_casecmp()
Improve command-line parser

Support prefix<any>, prefix=<any>, prefix <any>, [prefix<any>], [prefix=<any>]
and [prefix <any>] argument syntax forms.

Support [arg]... optional repeated argument syntax.
Improve keyring PIN handling

Replace explicit [<pin,pin...>] command-line arguments with --entry-pin=<pin>
options.  Update test scripts accordlingly.

Add --keyring-pin=<pin> option (with one test case).