__init_libc: add fallbacks for __progname setup

It is possible for argv[0] to be a null pointer, but the __progname
variable is used to implement functions in src/legacy/err.c that do not
expect it to be null. It is also available to the user via the
program_invocation_name alias as a GNU extension, and the implementation
in Glibc initializes it to a pointer to empty string rather than NULL.

Since argv[0] is usually non-null and it's preferable to keep those
variables in BSS, implement the fallbacks in __init_libc, which also
allows to have an intermediate fallback to AT_EXECFN.

src/env/__libc_start_main.c

@@ -30,10 +30,10 @@ void __init_libc(char **envp, char *pn)
 	__sysinfo = aux[AT_SYSINFO];
 	libc.page_size = aux[AT_PAGESZ];
 
-	if (pn) {
-		__progname = __progname_full = pn;
-		for (i=0; pn[i]; i++) if (pn[i]=='/') __progname = pn+i+1;
-	}
+	if (!pn) pn = (void*)aux[AT_EXECFN];
+	if (!pn) pn = "";
+	__progname = __progname_full = pn;
+	for (i=0; pn[i]; i++) if (pn[i]=='/') __progname = pn+i+1;
 
 	__init_tls(aux);
 	__init_ssp((void *)aux[AT_RANDOM]);