Use program exit status as pam_exec return code (optional)

pam_exec(8) now accepts a new option "return_prog_exit_status". When
set, the program exit status is used as the pam_exec return code. It
allows the program to tell why the step failed (eg. user unknown).
However, if it exits with a code not allowed by the calling PAM service
module function (see $PAM_SM_FUNC below), a warning is logged and
PAM_SERVICE_ERR is returned.

The following changes are related to this new feature but they apply no
matter if the "return_prog_exit_status" option is set or not.

The environment passed to the program is extended:
    o  $PAM_SM_FUNC contains the name of the PAM service module function
       (eg. pam_sm_authenticate).
    o  All valid PAM return codes' numerical values are available
       through variables named after the return code name. For instance,
       $PAM_SUCCESS, $PAM_USER_UNKNOWN or $PAM_PERM_DENIED.

pam_exec return code better reflects what went on:
    o  If the program exits with !0, the return code is now
       PAM_PERM_DENIED, not PAM_SYSTEM_ERR.
    o  If the program fails because of a signal (WIFSIGNALED) or doesn't
       terminate normally (!WIFEXITED), the return code is now
       PAM_SERVICE_ERR, not PAM_SYSTEM_ERR.
    o  If a syscall in pam_exec fails, the return code remains
       PAM_SYSTEM_ERR.

waitpid(2) is called in a loop. If it returns because of EINTR, do it
again. Before, it would return PAM_SYSTEM_ERR without waiting for the
child to exit.

Several log messages now include the PAM service module function name.

The man page is updated accordingly.

Reviewed by:	gleb@, des@
Sponsored by:	Yakaz (http://www.yakaz.com)
MFC after:	2 weeks

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