fish was indiscriminately calling `rustc -Z help` in the autocompletion
script for `rustc`, but `-Z` (and its `-Z help` output completions) is
only available when using the nightly compiler.
Note that this isn't a perfect fix since if you try to use those command
line options now added to the autocompletions list without using the
nightly toolchain, `rustc` will still throw an error. But at least this
way we don't cause random errors about `-Z` not being available to
appear any time someone tries to use `rustc` from the fish command prompt.
I believe apm must have been buggy - example output that I found online
showed `tr` was mangling paths with spaces in it. Should be fixed.
Also, use dscl on OS X in __fish_complete_users.fish like
__fish_print_users.fish already does.
