pam_mkhomedir: check and create home directory for the same user (ticket #22)

pam_mkhomedir: check and create home directory for the same user (ticket #22)

Before pam_mkhomedir helper was introduced in commit
7b14630ef39e71f603aeca0c47edf2f384717176, pam_mkhomedir was checking for
existance and creating the same directory - the home directory of the
user NAME returned by pam_get_item(PAM_USER).

The change in behaviour accidentally introduced along with
mkhomedir_helper is not consistent: while the module still checks for
getpwnam(NAME)->pw_dir, the directory created by mkhomedir_helper is
getpwnam(getpwnam(NAME)->pw_name)->pw_dir, which is not necessarily
the same as the directory being checked.

This change brings check and creation back in sync, both handling
getpwnam(NAME)->pw_dir.

* modules/pam_mkhomedir/pam_mkhomedir.c (create_homedir): Replace
"struct passwd *" argument with user's name and home directory.
Pass user's name to MKHOMEDIR_HELPER.
(pam_sm_open_session): Update create_homedir call.