LibC: Implement mkstemps() in stdlib and add a test

`mkstemps` generates a unique temporary file name from a pattern like
`prefixXXXXXXsuffix` where `prefix` and `suffix` can be any string with
only characters that are valid in a filename. The second parameter is
the length of the suffix.

`mkstemp` is `mkstemps` with suffix length 0, so to avoid code
duplication it calls `mkstemps`. It is unlikely this has any
significant performance impact on SerenityOS.

`generate_unique_filename` now takes the suffix length as a `size_t`.
The original behavior of this function is preserved when specifying a
suffix length of 0. All original uses of this function have been
adapted.

`mkstemps()` was added because it is required by version 4.6.3 of the
ccache port.

Tests/LibC/TestLibCMkTemp.cpp

@@ -121,3 +121,57 @@ TEST_CASE(test_mkstemp_unique_filename)
 
     munmap(ptr, sizeof(*ptr));
 }
+
+TEST_CASE(test_mkstemps_unique_filename)
+{
+    u8* ptr = (u8*)mmap(nullptr, 0x1000, PROT_READ | PROT_WRITE, MAP_SHARED | MAP_ANONYMOUS, -1, 0);
+    EXPECT_NE(ptr, MAP_FAILED);
+
+    if (fork() == 0) {
+        char path[] = "/tmp/test.mkstemps.prefixXXXXXXsuffix";
+        auto fd = mkstemps(path, 6);
+        EXPECT_NE(fd, -1);
+
+        auto temp_path_or_error = Core::File::read_link(String::formatted("/proc/{}/fd/{}", getpid(), fd));
+        EXPECT(!temp_path_or_error.is_error());
+
+        auto temp_path = temp_path_or_error.release_value();
+        EXPECT(temp_path.characters());
+
+        close(fd);
+        unlink(path);
+
+        EXPECT(temp_path.starts_with("/tmp/test.mkstemps.prefix"sv));
+        EXPECT(temp_path.ends_with("suffix"sv));
+        EXPECT_EQ(strlen(path), temp_path.length());
+
+        memcpy(&ptr[0], temp_path.characters(), temp_path.length());
+
+        exit(EXIT_SUCCESS);
+    } else {
+        wait(NULL);
+
+        auto path1 = String::formatted("{}", reinterpret_cast<char const*>(ptr));
+
+        char path[] = "/tmp/test.mkstemps.prefixXXXXXXsuffix";
+        auto fd = mkstemps(path, 6);
+        EXPECT(fd != -1);
+
+        auto path2_or_error = Core::File::read_link(String::formatted("/proc/{}/fd/{}", getpid(), fd));
+        EXPECT(!path2_or_error.is_error());
+
+        auto path2 = path2_or_error.release_value();
+        EXPECT(path2.characters());
+
+        close(fd);
+        unlink(path);
+
+        EXPECT(path2.starts_with("/tmp/test.mkstemps.prefix"sv));
+        EXPECT(path2.ends_with("suffix"sv));
+        EXPECT_EQ(strlen(path), path2.length());
+
+        EXPECT_NE(path1, path2);
+    }
+
+    munmap(ptr, sizeof(*ptr));
+}