os: fix os.execute for command output that contains 0 bytes

vlib/os/os_nix.c.v

@@ -343,16 +343,19 @@ pub fn execute(cmd string) Result {
 			output: 'exec("$cmd") failed'
 		}
 	}
+	fd := fileno(f)
 	buf := unsafe { malloc_noscan(4096) }
 	mut res := strings.new_builder(1024)
 	defer {
 		unsafe { res.free() }
 	}
 	unsafe {
-		bufbp := buf
-		for C.fgets(&char(bufbp), 4096, f) != 0 {
-			buflen := vstrlen(bufbp)
-			res.write_ptr(bufbp, buflen)
+		for {
+			len := C.read(fd, buf, 4096)
+			if len == 0 {
+				break
+			}
+			res.write_ptr(buf, len)
 		}
 	}
 	soutput := res.str()

vlib/os/os_test.v

@@ -841,3 +841,20 @@ fn test_expand_tilde_to_home() {
 	assert home_test == home_expansion_test
 	assert os.expand_tilde_to_home('~') == os.home_dir()
 }
+
+fn test_execute() {
+	$if !linux {
+		return
+	}
+	// The output of the next command contains a 0 byte in the middle.
+	// Nevertheless, the execute function *should* return a string that
+	// contains it:
+	result := os.execute('printenv --null LANGUAGE LANG')
+	hexresult := result.output.bytes().hex()
+	println(result.exit_code)
+	println(result.output.len)
+	println(hexresult)
+	assert result.exit_code == 0
+	assert result.output.len > 0
+	assert hexresult.contains('00')
+}