Clean up our OpenBSD usage.

Also undo some of the mess where we have OpenBSD <stdio.h> but a mix of
different BSD's implementations.

In this first pass, I've only moved easy OpenBSD stuff.

Change-Id: Iae67b02cde6dba9d8d06fedeb53efbfdac0a8cf6
diff --git a/libc/include/unistd.h b/libc/include/unistd.h
index 70cc4c5..d21f23d 100644
--- a/libc/include/unistd.h
+++ b/libc/include/unistd.h
@@ -64,6 +64,7 @@
 
 extern int execv(const char *, char * const *);
 extern int execvp(const char *, char * const *);
+extern int execvpe(const char *, char * const *, char * const *);
 extern int execve(const char *, char * const *, char * const *);
 extern int execl(const char *, const char *, ...);
 extern int execlp(const char *, const char *, ...);
@@ -194,7 +195,6 @@
 extern int   tcsetpgrp(int fd, pid_t _pid);
 
 #if 0 /* MISSING FROM BIONIC */
-extern int execvpe(const char *, char * const *, char * const *);
 extern int execlpe(const char *, const char *, ...);
 extern int getfsuid(uid_t);
 extern int setfsuid(uid_t);