use __strchrnul instead of strchr and strlen in execvpe

The result is the same but takes less code.
Note that __execvpe calls getenv which calls __strchrnul so even
using static output the size of the executable won't grow.

src/process/execvp.c

@@ -28,8 +28,7 @@ int __execvpe(const char *file, char *const argv[], char *const envp[])
 
 	for(p=path; ; p=z) {
 		char b[l+k+1];
-		z = strchr(p, ':');
-		if (!z) z = p+strlen(p);
+		z = __strchrnul(p, ':');
 		if (z-p >= l) {
 			if (!*z++) break;
 			continue;