Introduce pam_modutil_sanitize_helper_fds

Introduce pam_modutil_sanitize_helper_fds

This change introduces pam_modutil_sanitize_helper_fds - a new function
that redirects standard descriptors and closes all other descriptors.

pam_modutil_sanitize_helper_fds supports three types of input and output
redirection:
- PAM_MODUTIL_IGNORE_FD: do not redirect at all.
- PAM_MODUTIL_PIPE_FD: redirect to a pipe.  For stdin, it is implemented
  by creating a pipe, closing its write end, and redirecting stdin to
  its read end.  Likewise, for stdout/stderr it is implemented by
  creating a pipe, closing its read end, and redirecting to its write
  end.  Unlike stdin redirection, stdout/stderr redirection to a pipe
  has a side effect that a process writing to such descriptor should be
  prepared to handle SIGPIPE appropriately.
- PAM_MODUTIL_NULL_FD: redirect to /dev/null.  For stdin, it is
  implemented via PAM_MODUTIL_PIPE_FD because there is no functional
  difference.  For stdout/stderr, it is classic redirection to
  /dev/null.

PAM_MODUTIL_PIPE_FD is usually more suitable due to linux kernel
security restrictions, but when the helper process might be writing to
the corresponding descriptor and termination of the helper process by
SIGPIPE is not desirable, one should choose PAM_MODUTIL_NULL_FD.

* libpam/pam_modutil_sanitize.c: New file.
* libpam/Makefile.am (libpam_la_SOURCES): Add it.
* libpam/include/security/pam_modutil.h (pam_modutil_redirect_fd,
pam_modutil_sanitize_helper_fds): New declarations.
* libpam/libpam.map (LIBPAM_MODUTIL_1.1.9): New interface.
* modules/pam_exec/pam_exec.c (call_exec): Use
pam_modutil_sanitize_helper_fds.
* modules/pam_mkhomedir/pam_mkhomedir.c (create_homedir): Likewise.
* 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.
* modules/pam_xauth/pam_xauth.c (run_coprocess): Likewise.
* modules/pam_unix/support.h (MAX_FD_NO): Remove.