correct variadic prototypes for execl* family

the old versions worked, but conflicted with programs which declared
their own prototypes and generated warnings with some versions of gcc.

include/unistd.h

@@ -81,10 +81,10 @@ int pause(void);
 pid_t fork(void);
 int execve(const char *, char *const [], char *const []);
 int execv(const char *, char *const []);
-int execle(const char *, ...);
-int execl(const char *, ...);
+int execle(const char *, const char *, ...);
+int execl(const char *, const char *, ...);
 int execvp(const char *, char *const []);
-int execlp(const char *, ...);
+int execlp(const char *, const char *, ...);
 int fexecve(int, char *const [], char *const []);
 void _exit(int);
 

src/process/execl.c

 #include <unistd.h>
 #include <stdarg.h>
 
-int execl(const char *path, ...)
+int execl(const char *path, const char *argv0, ...)
 {
 	int argc;
 	va_list ap;
-	va_start(ap, path);
-	for (argc=0; va_arg(ap, const char *); argc++);
+	va_start(ap, argv0);
+	for (argc=1; va_arg(ap, const char *); argc++);
 	va_end(ap);
 	{
 		int i;
 		char *argv[argc+1];
-		va_start(ap, path);
-		for (i=0; i<argc; i++)
+		va_start(ap, argv0);
+		argv[0] = (char *)argv0;
+		for (i=1; i<argc; i++)
 			argv[i] = va_arg(ap, char *);
 		argv[i] = NULL;
 		return execv(path, argv);

src/process/execle.c

 #include <unistd.h>
 #include <stdarg.h>
 
-int execle(const char *path, ...)
+int execle(const char *path, const char *argv0, ...)
 {
 	int argc;
 	va_list ap;
-	va_start(ap, path);
-	for (argc=0; va_arg(ap, const char *); argc++);
+	va_start(ap, argv0);
+	for (argc=1; va_arg(ap, const char *); argc++);
 	va_end(ap);
 	{
 		int i;
 		char *argv[argc+1];
 		char **envp;
-		va_start(ap, path);
-		for (i=0; i<argc; i++)
+		va_start(ap, argv0);
+		argv[0] = (char *)argv0;
+		for (i=1; i<argc; i++)
 			argv[i] = va_arg(ap, char *);
 		argv[i] = NULL;
 		envp = va_arg(ap, char **);

src/process/execlp.c

 #include <unistd.h>
 #include <stdarg.h>
 
-int execlp(const char *file, ...)
+int execlp(const char *file, const char *argv0, ...)
 {
 	int argc;
 	va_list ap;
-	va_start(ap, file);
-	for (argc=0; va_arg(ap, const char *); argc++);
+	va_start(ap, argv0);
+	for (argc=1; va_arg(ap, const char *); argc++);
 	va_end(ap);
 	{
 		int i;
 		char *argv[argc+1];
-		va_start(ap, file);
-		for (i=0; i<argc; i++)
+		va_start(ap, argv0);
+		argv[0] = (char *)argv0;
+		for (i=1; i<argc; i++)
 			argv[i] = va_arg(ap, char *);
 		argv[i] = NULL;
 		return execvp(file, argv);