unbreak tree for openbsd after rust-lang#21787

- add `_SC_GETPW_R_SIZE_MAX` constant
- declare `struct passwd`
- convert `load_self` to `current_exe`

Note: OpenBSD don't provide system function to return a valuable Path
for `env::current_exe`. The implementation is currently based on the
value of `argv[0]`, which couldn't be used when executable is called via
PATH.

src/libstd/sys/unix/c.rs

@@ -74,6 +74,8 @@ pub const _SC_GETPW_R_SIZE_MAX: libc::c_int = 70;
 #[cfg(any(target_os = "macos",
           target_os = "freebsd"))]
 pub const _SC_GETPW_R_SIZE_MAX: libc::c_int = 71;
+#[cfg(target_os = "openbsd")]
+pub const _SC_GETPW_R_SIZE_MAX: libc::c_int = 101;
 #[cfg(target_os = "android")]
 pub const _SC_GETPW_R_SIZE_MAX: libc::c_int = 0x0048;
 
@@ -91,7 +93,8 @@ pub struct passwd {
 
 #[repr(C)]
 #[cfg(any(target_os = "macos",
-          target_os = "freebsd"))]
+          target_os = "freebsd",
+          target_os = "openbsd"))]
 pub struct passwd {
     pub pw_name: *mut libc::c_char,
     pub pw_passwd: *mut libc::c_char,

src/libstd/sys/unix/os.rs

@@ -197,23 +197,23 @@ pub fn current_exe() -> IoResult<Path> {
 }
 
 #[cfg(target_os = "openbsd")]
-pub fn load_self() -> Option<Vec<u8>> {
+pub fn current_exe() -> IoResult<Path> {
     use sync::{StaticMutex, MUTEX_INIT};
 
     static LOCK: StaticMutex = MUTEX_INIT;
 
     extern {
-        fn rust_load_self() -> *const c_char;
+        fn rust_current_exe() -> *const c_char;
     }
 
     let _guard = LOCK.lock();
 
     unsafe {
-        let v = rust_load_self();
+        let v = rust_current_exe();
         if v.is_null() {
-            None
+            Err(IoError::last_error())
         } else {
-            Some(ffi::c_str_to_bytes(&v).to_vec())
+            Ok(Path::new(ffi::c_str_to_bytes(&v).to_vec()))
         }
     }
 }
@@ -333,7 +333,8 @@ pub fn args() -> Args {
 #[cfg(any(target_os = "linux",
           target_os = "android",
           target_os = "freebsd",
-          target_os = "dragonfly"))]
+          target_os = "dragonfly",
+          target_os = "openbsd"))]
 pub fn args() -> Args {
     use rt;
     let bytes = rt::args::clone().unwrap_or(Vec::new());

src/rt/rust_builtin.c

@@ -204,7 +204,7 @@ int *__dfly_error(void) { return __error(); }
 #include <sys/sysctl.h>
 #include <limits.h>
 
-const char * rust_load_self() {
+const char * rust_current_exe() {
     static char *self = NULL;
 
     if (self == NULL) {