drop use of stat operation in temporary file name generation

this change serves two purposes:

1. it eliminates one of the few remaining uses of the kernel stat
structure which will not be present in future archs, avoiding the need
for growing ifdef logic here.

2. it potentially makes the operations less expensive when the
candidate exists as a non-symlink by avoiding the need to read the
inode (assuming the directory tables suffice to distinguish symlinks).

this uses the idiom I discovered while rewriting realpath for commit
29ff7599a4 of being able to use the
readlink operation as an inexpensive probe for file existence that
doesn't following symlinks.

src/stdio/tempnam.c

@@ -6,7 +6,6 @@
 #include <string.h>
 #include <stdlib.h>
 #include "syscall.h"
-#include "kstat.h"
 
 #define MAXTRIES 100
 
@@ -37,11 +36,10 @@ char *tempnam(const char *dir, const char *pfx)
 
 	for (try=0; try<MAXTRIES; try++) {
 		__randname(s+l-6);
-#ifdef SYS_lstat
-		r = __syscall(SYS_lstat, s, &(struct kstat){0});
+#ifdef SYS_readlink
+		r = __syscall(SYS_readlink, s, (char[1]){0}, 1);
 #else
-		r = __syscall(SYS_fstatat, AT_FDCWD, s,
-			&(struct kstat){0}, AT_SYMLINK_NOFOLLOW);
+		r = __syscall(SYS_readlinkat, AT_FDCWD, s, (char[1]){0}, 1);
 #endif
 		if (r == -ENOENT) return strdup(s);
 	}

src/stdio/tmpnam.c

@@ -5,7 +5,6 @@
 #include <string.h>
 #include <stdlib.h>
 #include "syscall.h"
-#include "kstat.h"
 
 #define MAXTRIES 100
 
@@ -17,11 +16,10 @@ char *tmpnam(char *buf)
 	int r;
 	for (try=0; try<MAXTRIES; try++) {
 		__randname(s+12);
-#ifdef SYS_lstat
-		r = __syscall(SYS_lstat, s, &(struct kstat){0});
+#ifdef SYS_readlink
+		r = __syscall(SYS_readlink, s, (char[1]){0}, 1);
 #else
-		r = __syscall(SYS_fstatat, AT_FDCWD, s,
-			&(struct kstat){0}, AT_SYMLINK_NOFOLLOW);
+		r = __syscall(SYS_readlinkat, AT_FDCWD, s, (char[1]){0}, 1);
 #endif
 		if (r == -ENOENT) return strcpy(buf ? buf : internal, s);
 	}