login-utils, sys-utils: use _PATH_BSHELL consistently

login-utils, sys-utils: use _PATH_BSHELL consistently

Remove local DEFAULT_SHELL definitions and hardcoded "/bin/sh" strings
in favor of the standard _PATH_BSHELL macro from <paths.h>. This
provides consistency across the codebase while following libc
conventions.

These tools already perform their own passwd lookups and only need a
fallback value, so they don't require the full ul_default_shell()
resolution logic.

Affected tools:
- su(1): already checks pw_shell validity
- sulogin(8): emergency login with explicit shell handling
- setpriv(1): already has passwd entry for environment setup

Signed-off-by: Alessandro Ratti <alessandro@0x65c.net>