This PR provides support for the select() system call:

• adds new syscall handler in Shadow so native process calls to select no longer fail.
• re-enables legacy TCP test that can optionally use select as an event monitor.
• adds a new select-specific test to more completely test the select API against linux.

I decided to implement the handler in C first - it internally uses some code from our poll handler whose functionality is largely similar. I'll loop around to convert both the poll and select handlers to rust.

Closes #1721