(run_shell): Pass a new n_additional_args arg, so that

(run_shell): Pass a new n_additional_args arg, so that
the callee doesn't have to count 'em.  All callers changed.
Don't allocate more space for the arg vector than we'll need.
Use memcpy to copy the args rather than rolling our own loop.
Use size_t for sizes.
(fast_startup, simulate_login, change_environment, log_su,
correct_password, restricted_shell, main): Use bool for booleans.
(longopts): Don't assume change_environment is an int.
Use NULL, not 0, for pointers.
(xsetenv): New function, replacing xputenv and concat.
All callers changed.
(elements): Remove; no longer needed.
(log_su, correct_passwd, main): Prefer !x to x==NULL.
(log_su): 2 -> STDERR_FILENO.
(modify_environment, main): Don't assume that getenv's returned value
has an indefinite lifetime.
(modify_environment): Allocate a larger environ.
(main): Remove an impossible 'case 0'; if it happens now, it'll
get diagnosed.  Don't assume getpwnam results outlive endpwent.
Check for null or empty pw_name, pw_dir and for null pw_passwd.