Only invoke path_helper in login shells

[thefloweringash]

Matches upstream path_helper which is invoked in /etc/profile and only
applies to login shells. Enables running interactive, non-login shells
with altered PATH values.

Reverts change in c0f832a, which reverts change in adbaddf.

Description

The macOS path_helper prepends all system specified paths:

$ PATH=/foo /usr/libexec/path_helper -s
PATH="/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin:/opt/X11/bin:/foo"; export PATH;

It's sometimes useful to run a shell with an altered path, for example to inject custom entries ahead of, or instead of, system entries. On bash this is possible since the path_helper is only run in a login shell:

$ PATH=/foo `which bash` -i -c 'echo $PATH'
/foo

$ PATH=/foo `which bash` -l -i -c 'echo $PATH'
/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin:/opt/X11/bin:/foo

This makes fish behave equivalently.

TODOs:

• Changes to fish usage are reflected in user documentation/manpages.
• Tests have been added for regressions fixed
• User-visible changes noted in CHANGELOG.md

[ridiculousfish]

Looks good, now it's justified :) Please update CHANGELOG.md

[thefloweringash]

Added a brief summary to the 3.0.1 fixes section.

[ridiculousfish]

merged as 56309f1, thanks!!

Note this is going into 3.1.0, not 3.0.1 (I updated the changelog appropriately)