src/: Fix uninitialized flags, and use const appropriately

src/: Fix uninitialized flags, and use const appropriately

In all these functions, we were setting the flags to 'true' in
process_flags() if the appropriate command-line flag was specified.
However, they were never being defined to 'false', which should be the
default value.  Initialize these flags to false.

Fixes: c0c9485d9a5a (2025-10-07; "src/useradd.c: chroot or prefix SELinux file context")
Link: <https://github.com/shadow-maint/shadow/pull/1396>
Reviewed-by: Serge Hallyn <serge@hallyn.com>
Cc: Iker Pedrosa <ipedrosa@redhat.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>