Auto merge of #2914 - SteveLauC:Wrong-getpwent_r-definition-on-solarish-os, r=JohnTitor

fix wrong definitions of getpwent_r and getgrent_r on solarish os

Closes #2908

* [man page for `getpwent_r`](https://illumos.org/man/3C/getpwnam)
* [man page for `getgrent_r`](https://illumos.org/man/3C/getgrnam)

You may find the definitions for `getpwnam_r/getpwuid_r/getgrnam_r/getgruid_r` exposed by `libc` are also wrong:
```c
struct passwd *getpwnam_r(const char *name, struct passwd *pwd,
            char *buffer, int buflen);
```
```rust
pub fn getpwnam_r(
    name: *const ::c_char,
    pwd: *mut passwd,
    buf: *mut ::c_cha
    buflen: ::size_t,
    result: *mut *mut passwd,
) -> ::c_int;
```
But actually they are **correct** as there are the POSIX-conforming definitions (see `Standard conforming` section of above man pages):
```
Standard conforming

       cc [ flag...] file... -D_POSIX_PTHREAD_SEMANTICS [ library... ]

       int getpwnam_r(const char *name, struct passwd *pwd, char *buffer,
            size_t bufsize, struct passwd **result);

       int getpwuid_r(uid_t uid, struct passwd *pwd, char *buffer,
            size_t bufsize, struct passwd **result);
```

`getpwent_r/getgrent_r` don't get lucky, they do not have the POSIX-conforming alternatives.

To double check this, I searched its [source code](https://github.com/illumos/illumos-gate/blob/master/usr/src/lib/libc/port/gen/getpwnam_r.c):
```shell
$ rg "__posix_getpwnam_r"
port/mapfile-vers
1582:   __posix_getpwnam_r;

port/gen/getpwnam_r.c
152:__posix_getpwnam_r(const char *name, struct passwd *pwd, char *buffer,

$ rg "__posix_getpwent_r"

$
```

Author: bors

src/unix/solarish/compat.rs

@@ -1,6 +1,7 @@
 // Common functions that are unfortunately missing on illumos and
 // Solaris, but often needed by other crates.
 
+use core::cmp::min;
 use unix::solarish::*;
 
 const PTEM: &[u8] = b"ptem\0";
@@ -169,3 +170,51 @@ pub unsafe fn forkpty(
 
     0
 }
+
+pub unsafe fn getpwent_r(
+    pwd: *mut passwd,
+    buf: *mut ::c_char,
+    buflen: ::size_t,
+    result: *mut *mut passwd,
+) -> ::c_int {
+    let old_errno = *::___errno();
+    *::___errno() = 0;
+    *result = native_getpwent_r(
+        pwd,
+        buf,
+        min(buflen, ::c_int::max_value() as ::size_t) as ::c_int,
+    );
+
+    let ret = if (*result).is_null() {
+        *::___errno()
+    } else {
+        0
+    };
+    *::___errno() = old_errno;
+
+    ret
+}
+
+pub unsafe fn getgrent_r(
+    grp: *mut ::group,
+    buf: *mut ::c_char,
+    buflen: ::size_t,
+    result: *mut *mut ::group,
+) -> ::c_int {
+    let old_errno = *::___errno();
+    *::___errno() = 0;
+    *result = native_getgrent_r(
+        grp,
+        buf,
+        min(buflen, ::c_int::max_value() as ::size_t) as ::c_int,
+    );
+
+    let ret = if (*result).is_null() {
+        *::___errno()
+    } else {
+        0
+    };
+    *::___errno() = old_errno;
+
+    ret
+}

src/unix/solarish/mod.rs

@@ -3016,24 +3016,14 @@ extern "C" {
     ) -> ::c_int;
     #[cfg_attr(
         any(target_os = "solaris", target_os = "illumos"),
-        link_name = "__posix_getpwent_r"
+        link_name = "getpwent_r"
     )]
-    pub fn getpwent_r(
-        pwd: *mut passwd,
-        buf: *mut ::c_char,
-        buflen: ::size_t,
-        result: *mut *mut passwd,
-    ) -> ::c_int;
+    fn native_getpwent_r(pwd: *mut passwd, buf: *mut ::c_char, buflen: ::c_int) -> *mut passwd;
     #[cfg_attr(
         any(target_os = "solaris", target_os = "illumos"),
-        link_name = "__posix_getgrent_r"
+        link_name = "getgrent_r"
     )]
-    pub fn getgrent_r(
-        grp: *mut ::group,
-        buf: *mut ::c_char,
-        buflen: ::size_t,
-        result: *mut *mut ::group,
-    ) -> ::c_int;
+    fn native_getgrent_r(grp: *mut ::group, buf: *mut ::c_char, buflen: ::c_int) -> *mut ::group;
     #[cfg_attr(
         any(target_os = "solaris", target_os = "illumos"),
         link_name = "__posix_sigwait"