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/shadowlog.c

@@ -2,14 +2,17 @@
 
 #include "lib/shadowlog_internal.h"
 
+const char *shadow_progname;
+FILE *shadow_logfd;
+
 void log_set_progname(const char *progname)
 {
-	Prog = progname;
+	shadow_progname = progname;
 }
 
 const char *log_get_progname(void)
 {
-	return Prog;
+	return shadow_progname;
 }
 
 void log_set_logfd(FILE *fd)