maint: refactor SMACK interface to a separate module

maint: refactor SMACK interface to a separate module

Consolidate all smack routines and checks in a module.
We replace and wrap the most commonly used smack routines,
which allows removing ifdefs throughout the code.

* gl/lib/smack.h: A new header containing the implementation
of the wrapped and replacement routines.  Note the is_smack_enabled()
routine should be optimized out at compile time when compiled
on a system without libsmack.
* gl/modules/smack: Describe the new module and move the
configure time code here from ...
* m4/jm-macros.m4: ... here.
* bootstrap.conf: Reference the new module.
* src/id.c: Use the routines without ifdefs where possible.
* src/ls.c: Likewise.
* src/mkdir.c: Likewise.
* src/mkfifo.c: Likewise.
* src/mknod.c: Likewise.