lib: rename Prog to shadow_progname, with only one definition

The build was failing with duplicate symbol errors with -fno-common.
This is the default in GCC 10 and later, and explicitly enabled in some
distributions to catch problems like this. There were two causes:

- Prog and shadow_logfd were defined in a header file that was included
  in multiple other files. Fix this by defining them once in
  shadowlog.c, and having extern declarations in the header.

- Most of the tools (except id/nologin) also define a Prog variable,
  which is not intended to alias the one in the library. Fix
  this by renaming Prog in the library to shadow_progname, which also
  matches the new accessor functions for it.

lib/selinux.c

@@ -216,7 +216,7 @@ int check_selinux_permit (const char *perm_name)
 	if (getprevcon_raw (&user_context_raw) != 0) {
 		fprintf (shadow_logfd,
 		    _("%s: can not get previous SELinux process context: %s\n"),
-		    Prog, strerror (errno));
+		    shadow_progname, strerror (errno));
 		SYSLOG ((LOG_WARN,
 		    "can not get previous SELinux process context: %s",
 		    strerror (errno)));