interp: support pwd flags like -P

[amancevice]

I use this repo indirectly through go-task and noticed an unexpected result when executing the command pwd -P. The issue is outlined in go-task#533.

In essence, when executed from a symlinked directory, pwd -P outputs the symlink and not the actual physical location on disk.

Example:

mkdir real-dir
ln -s real-dir link-dir
cd link-dir
pwd -P
# => Expected: /path/to/real-dir
# => Actual: /path/to/link-dir

[mvdan]

Ah, that's sort-of-expected given that we don't support any pwd flags right now. See:

sh/interp/builtin.go

Lines 186 to 187 in ec61336

	case "pwd":
	r.outf("%s\n", r.envGet("PWD"))

I think it should be doable with https://pkg.go.dev/path/filepath#EvalSymlinks and a couple of tests, if you're up for sending a PR.

[amancevice]

I'm a Go-newbie but I can give it the old college try in the coming days. Thanks!