Add std/cli, move std/getopt there

[fare]

Import a bunch of CLI utilities from gerbil-utils into std/cli; tweak them, document them, test them:

• multicall to support multiple subcommands in a more modular way
• getopt translation to Scheme calling convention
• print-exit to print results of Scheme computation at the CLI
• shell to support escaping strings for the shell

Add a few corresponding utilities to std/error.

Move std/getopt to std/cli/getopt with its brothers; leave a compatibility shim at std/getopt.

Use std/cli/getopt everywhere instead of std/getopt.

Update documentation.

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[fare]

Replaces #1027

[vyzo]

build fails

[fare]

It was a rebasing issue (I was using fx>=0?). Fixed.

[fare]

Also avoiding defmutable for now. Sigh.

[fare]

PTAL