sys/posix: impl rest of linux, impl some of Windows

core/sys/posix/pwd.odin

@@ -1,3 +1,4 @@
+#+build linux, darwin, netbsd, openbsd, freebsd
 package posix
 
 import "core:c"
@@ -163,6 +164,16 @@ when ODIN_OS == .Darwin || ODIN_OS == .NetBSD || ODIN_OS == .OpenBSD {
 		pw_fields: c.int,
 	}
 
-} else {
-	#panic("posix is unimplemented for the current target")
+} else when ODIN_OS == .Linux {
+
+	passwd :: struct {
+		pw_name:   cstring, /* [PSX] user name */
+		pw_passwd: cstring, /* encrypted password */
+		pw_uid:    uid_t,   /* [PSX] user uid */
+		pw_gid:    gid_t,   /* [PSX] user gid */
+		pw_gecos:  cstring, /* Real name.  */
+		pw_dir:    cstring, /* Home directory.  */
+		pw_shell:  cstring, /* Shell program.  */
+	}
+
 }