What should relative paths be relative to in the JS shell?

[Steve Fink]

Over in bug 835552, I'm trying to change the behavior of relative path
lookups.

Current behavior:
- load("foo/bar.js") will search relative to the current working directory
- read("foo/bar.txt") will search relative to the calling script's
filename on XP_UNIX (and relative to the current working directory on
non-XP_UNIX, but ignore that for now.)

To me, that seems backwards. I want to be able to be able to have a
directory of scripts that load each other, so load() needs to be
caller-relative. And the actual reason I'm doing something with this is
because I want to be able to do

system("something > tmp.txt"); load("tmp.txt")

and have it find the file. It would also be nice to be able to run

js ~/scripts/process.js somefile.txt

(where process.js calls read(arguments[0])) and have the JS shell
resolve the path the same way that bash would -- ie, relative to the cwd.

But there are uses for the current behavior. I've added a
readRelativeToScript() so you can have data files alongside scripts. Now
in https://bugzilla.mozilla.org/show_bug.cgi?id=835552#c15 njn argues
that the current cwd-relative behavior of load() should be the default
and there should be a loadRelativeToScript() available.

Note that probably a better solution would be to implement some basic
path manipulation routines and perhaps a nicer way to get the current
script's filename, but I've already scope-creeped this and this question
is really about the default behavior.

Opinions?