su: suppress PAM info messages for -c or non-login sessions

su: suppress PAM info messages for -c or non-login sessions

The 'Last login:' messages from PAM lastlogin module is unexpected
for non-login sessions or when -c <command> executed.

For example:

  $ su - -c id
  Last login: Wed Jul 24 08:36:28 CEST 2013 from dhcp-25-161.brq.redhat.com on pts/18
  uid=0(root) gid=0(root) skupiny=0(root)

this makes 'su' useless in scripts.

This patch suppress all PAM_TEXT_INFO messages for -c and for
non-login session ('-' is not specified) after pam_authenticate() and
pam_acct_mgmt().

Note that the new PAM conversation function checks the first message
in the msg[] array only. It seems good enough as PAM internally uses
pam_info() function that does not use multiple messages for one conv
call.

References: https://bugzilla.redhat.com/show_bug.cgi?id=987787
Signed-off-by: Karel Zak <kzak@redhat.com>