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/spawn.c

@@ -60,11 +60,11 @@ int run_command (const char *cmd, const char *argv[],
 			exit (E_CMD_NOTFOUND);
 		}
 		fprintf (shadow_logfd, "%s: cannot execute %s: %s\n",
-		         Prog, cmd, strerror (errno));
+		         shadow_progname, cmd, strerror (errno));
 		exit (E_CMD_NOEXEC);
 	} else if ((pid_t)-1 == pid) {
 		fprintf (shadow_logfd, "%s: cannot execute %s: %s\n",
-		         Prog, cmd, strerror (errno));
+		         shadow_progname, cmd, strerror (errno));
 		return -1;
 	}
 
@@ -77,7 +77,7 @@ int run_command (const char *cmd, const char *argv[],
 
 	if ((pid_t)-1 == wpid) {
 		fprintf (shadow_logfd, "%s: waitpid (status: %d): %s\n",
-		         Prog, *status, strerror (errno));
+		         shadow_progname, *status, strerror (errno));
 		return -1;
 	}