Align function prototypes for libss's request handler function

Align function prototypes for libss's request handler function

Clang 17's Undefined Behaviour Sanitizer will throw run-time warnings
if a function pointer is dereferenced with a different function
signature than one in the pointer --- even if the difference is a
missing const qualifier.  To fix regression test failures, change
declarations of argv to use ss_argv_t instead of an inconsistently
open-coded type.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>