river: remove system /etc dir from init search paths

A true "default" config doesn't make sense for river. Everyone who uses
river seriously will customize their init script. Futhermore, the
current behavior of embedding the install path of the default system
config in the river binary is complex and prone to breaking.

river/main.zig

@@ -16,6 +16,7 @@
 // along with this program. If not, see <https://www.gnu.org/licenses/>.
 
 const std = @import("std");
+const fs = std.fs;
 const os = std.os;
 const wlr = @import("wlroots");
 
@@ -86,9 +87,7 @@ pub fn main() anyerror!void {
         .warn, .err, .crit, .alert, .emerg => .err,
     });
 
-    if (startup_command == null) {
-        if (try getStartupCommand()) |path| startup_command = path;
-    }
+    if (startup_command == null) startup_command = try defaultInitPath();
 
     std.log.info("initializing server", .{});
     try server.init();
@@ -99,7 +98,7 @@ pub fn main() anyerror!void {
     // Run the child in a new process group so that we can send SIGTERM to all
     // descendants on exit.
     const child_pgid = if (startup_command) |cmd| blk: {
-        std.log.info("running startup command '{s}'", .{cmd});
+        std.log.info("running init executable '{s}'", .{cmd});
         const child_args = [_:null]?[*:0]const u8{ "/bin/sh", "-c", cmd, null };
         const pid = try os.fork();
         if (pid == 0) {
@@ -111,8 +110,9 @@ pub fn main() anyerror!void {
         // Since the child has called setsid, the pid is the pgid
         break :blk pid;
     } else null;
-    defer if (child_pgid) |pgid|
-        os.kill(-pgid, os.SIGTERM) catch |e| std.log.err("failed to kill startup process: {s}", .{e});
+    defer if (child_pgid) |pgid| os.kill(-pgid, os.SIGTERM) catch |err| {
+        std.log.err("failed to kill init process group: {s}", .{@errorName(err)});
+    };
 
     std.log.info("running server", .{});
 
@@ -127,29 +127,24 @@ fn printErrorExit(comptime format: []const u8, args: anytype) noreturn {
     os.exit(1);
 }
 
-fn testConfigPath(comptime fmt: []const u8, args: anytype) std.fmt.AllocPrintError!?[:0]const u8 {
-    const path = try std.fmt.allocPrintZ(util.gpa, fmt, args);
-    os.access(path, os.X_OK) catch {
+fn defaultInitPath() !?[:0]const u8 {
+    const path = blk: {
+        if (os.getenv("XDG_CONFIG_HOME")) |xdg_config_home| {
+            break :blk try fs.path.joinZ(util.gpa, &[_][]const u8{ xdg_config_home, "river/init" });
+        } else if (os.getenv("HOME")) |home| {
+            break :blk try fs.path.joinZ(util.gpa, &[_][]const u8{ home, ".config/river/init" });
+        } else {
+            return null;
+        }
+    };
+
+    os.accessZ(path, os.X_OK) catch |err| {
+        std.log.err("failed to run init executable {s}: {s}", .{ path, @errorName(err) });
         util.gpa.free(path);
         return null;
     };
-    return path;
-}
 
-fn getStartupCommand() std.fmt.AllocPrintError!?[:0]const u8 {
-    if (os.getenv("XDG_CONFIG_HOME")) |xdg_config_home| {
-        if (try testConfigPath("{s}/river/init", .{xdg_config_home})) |path| {
-            return path;
-        }
-    } else if (os.getenv("HOME")) |home| {
-        if (try testConfigPath("{s}/.config/river/init", .{home})) |path| {
-            return path;
-        }
-    }
-    if (try testConfigPath(build_options.default_config_path, .{})) |path| {
-        return path;
-    }
-    return null;
+    return path;
 }
 
 pub fn log(