Prior to this change, inherited environment variables
would be split on colons, becoming an array. This change
eliminates that behavior. Now environment variables are
always split on the record separator character (ASCII 0x1e),
with the exception of a short whitelist of PATH, MANPATH,
CDPATH. Likewise, exported variables are also exported
delimited by rs, with the exception of the above whitelist.
Fixes#1374, also see #1656
We can't use $PATHS to test the :-splitting because the global config
file adds extra paths based on /etc/paths and /etc/paths.d.
Ideally fish would have a way to suppress behavior like that, but for
the time being it doesn't.
`exec` removes fish from the shell "stack", so SHLVL needs to be
decremented to match. This means `exec fish` will result in the same
SHLVL in the new fish instance.
Also tweak the SHLVL logic to interpret an environment SHLVL of "3foo"
as garbage instead of as the value "3".
Fixes#1693.
