Auto merge of #481 - philippkeller:master, r=alexcrichton

Add execl, execle, execlp

I tried to be as close as possible to the existing definitions of `execv` and friends.

btw: do you still welcome these PR? Doesn't the [new bindgen/build.rs method](http://fitzgeraldnick.com/2016/12/14/using-libbindgen-in-build-rs.html) offer to automate this crate to 100%

Author: bors

src/unix/mod.rs

@@ -377,6 +377,12 @@ extern {
     pub fn close(fd: ::c_int) -> ::c_int;
     pub fn dup(fd: ::c_int) -> ::c_int;
     pub fn dup2(src: ::c_int, dst: ::c_int) -> ::c_int;
+    pub fn execl(path: *const c_char,
+                 arg0: *const c_char, ...) -> ::c_int;
+    pub fn execle(path: *const ::c_char,
+                  arg0: *const ::c_char, ...) -> ::c_int;
+    pub fn execlp(file: *const ::c_char,
+                  arg0: *const ::c_char, ...) -> ::c_int;
     pub fn execv(prog: *const c_char,
                  argv: *const *const c_char) -> ::c_int;
     pub fn execve(prog: *const c_char, argv: *const *const c_char,