Fix gratuitous use of strdup and x_strdup

Fix gratuitous use of strdup and x_strdup

There is no need to copy strings passed as arguments to execve,
the only potentially noticeable effect of using strdup/x_strdup
would be a malformed argument list in case of memory allocation error.

Also, x_strdup, being a thin wrapper around strdup, is of no benefit
when its argument is known to be non-NULL, and should not be used in
such cases.

* modules/pam_cracklib/pam_cracklib.c (password_check): Use strdup
instead of x_strdup, the latter is of no benefit in this case.
* modules/pam_ftp/pam_ftp.c (lookup): Likewise.
* modules/pam_userdb/pam_userdb.c (user_lookup): Likewise.
* modules/pam_userdb/pam_userdb.h (x_strdup): Remove.
* modules/pam_mkhomedir/pam_mkhomedir.c (create_homedir): Do not use
x_strdup for strings passed as arguments to execve.
* modules/pam_unix/pam_unix_acct.c (_unix_run_verify_binary): Likewise.
* modules/pam_unix/pam_unix_passwd.c (_unix_run_update_binary): Likewise.
* modules/pam_unix/support.c (_unix_run_helper_binary): Likewise.
(_unix_verify_password): Use strdup instead of x_strdup, the latter
is of no benefit in this case.
* modules/pam_xauth/pam_xauth.c (run_coprocess): Do not use strdup for
strings passed as arguments to execv.