util::opts::print_usage panics on broken pipe

[TorstenScheck]

Not a major use case, but not nice nevertheless.

How to reproduce:

$ ./mach run -r -- --help | :

thread 'main' panicked at 'failed printing to stdout: Broken pipe (os error 32)', ../src/libstd/io/stdio.rs:617
stack backtrace:
   1: 0xb6e642a4 - std::sys::backtrace::tracing::imp::write::h6528da8103c51ab9
   2: 0xb6e6c571 - std::panicking::default_hook::_$u7b$$u7b$closure$u7d$$u7d$::hbe741a5cc3c49508
   3: 0xb6e6c176 - std::panicking::default_hook::he0146e6a74621cb4
   4: 0xb6e51261 - std::panicking::rust_panic_with_hook::h983af77c1a2e581b
   5: 0xb6e6c843 - std::panicking::begin_panic::he426e15a3766089a
   6: 0xb6e52d6b - std::panicking::begin_panic_fmt::hdddb415186c241e7
   7: 0xb6e5907c - std::io::stdio::_print::h4cd911b6be2080b8
   8: 0xb69a02a7 - util::opts::print_usage::h4625d5a61b8d078d
   9: 0xb69a91fb - util::opts::from_cmdline_args::h9a0ca4c30dd73e45
  10: 0xb4efc517 - servo::main::h6924e5658370f6b5
  11: 0xb6e6baf5 - std::panicking::try::call::h852b0d5f2eec25e4
  12: 0xb6e75567 - __rust_try
  13: 0xb6e754fd - __rust_maybe_catch_panic
  14: 0xb6e6b58f - std::rt::lang_start::hfe4efe1fc39e4a30
  15: 0xb4f5413d - main
  16: 0xb459eaf2 - __libc_start_main
  17: 0xb4efb83c - <unknown>

[jdm]

I don't think there's anything we can do about this, since the stdio stdout printing functions internally panic when errors are encountered.

[nox]

@jdm We could use write!.