[os] Darwin,FreeBSD,OpenBSD: Rename os.getenv to os.get_env

Make os.get_env consistent across Unixes

This matches the function name and API from env_windows.odin and os_linux.odin, which should be the same everywhere. Meaning:

* named get_env and not getenv
* return a string (empty if the environment variable is not found)
* accept a default value parameter for the allocator (defaulting to context.allocator)
* calls lookup_env which returns an extra found boolean value

This is so that you don't have to write platform/OS conditionals when getting environment variable values from the stdlib os.get_env/getenv function.

core/os/os_darwin.odin

@@ -633,13 +633,18 @@ heap_free :: proc(ptr: rawptr) {
 	_unix_free(ptr)
 }
 
-getenv :: proc(name: string) -> (string, bool) {
-	path_str := strings.clone_to_cstring(name, context.temp_allocator)
+lookup_env :: proc(key: string, allocator := context.allocator) -> (value: string, found: bool) {
+	path_str := strings.clone_to_cstring(key, context.temp_allocator)
 	cstr := _unix_getenv(path_str)
 	if cstr == nil {
 		return "", false
 	}
-	return string(cstr), true
+	return strings.clone(string(cstr), allocator), true
+}
+
+get_env :: proc(key: string, allocator := context.allocator) -> (value: string) {
+	value, _ = lookup_env(key, allocator)
+	return
 }
 
 get_current_directory :: proc() -> string {

core/os/os_freebsd.odin

@@ -618,13 +618,18 @@ heap_free :: proc(ptr: rawptr) {
 	_unix_free(ptr)
 }
 
-getenv :: proc(name: string) -> (string, bool) {
-	path_str := strings.clone_to_cstring(name, context.temp_allocator)
+lookup_env :: proc(key: string, allocator := context.allocator) -> (value: string, found: bool) {
+	path_str := strings.clone_to_cstring(key, context.temp_allocator)
 	cstr := _unix_getenv(path_str)
 	if cstr == nil {
 		return "", false
 	}
-	return string(cstr), true
+	return strings.clone(string(cstr), allocator), true
+}
+
+get_env :: proc(key: string, allocator := context.allocator) -> (value: string) {
+	value, _ = lookup_env(key, allocator)
+	return
 }
 
 get_current_directory :: proc() -> string {

core/os/os_openbsd.odin

@@ -620,13 +620,18 @@ heap_free :: proc(ptr: rawptr) {
 	_unix_free(ptr)
 }
 
-getenv :: proc(name: string) -> (string, bool) {
-	path_str := strings.clone_to_cstring(name, context.temp_allocator)
+lookup_env :: proc(key: string, allocator := context.allocator) -> (value: string, found: bool) {
+	path_str := strings.clone_to_cstring(key, context.temp_allocator)
 	cstr := _unix_getenv(path_str)
 	if cstr == nil {
 		return "", false
 	}
-	return string(cstr), true
+	return strings.clone(string(cstr), allocator), true
+}
+
+get_env :: proc(key: string, allocator := context.allocator) -> (value: string) {
+	value, _ = lookup_env(key, allocator)
+	return
 }
 
 get_current_directory :: proc() -> string {