rc.subr: Support loading environmental variables from a file

[feld]

Summary

The ${name}_env is not convenient for setting multiple environmental variables
or variables that may have spaces in the value field. Support loading
environment from a file.

[feld]

Fix re-enabling +a

[feld]

When the env file is missing it breaks the rc script in a fairly obvious way:

.: cannot open /usr/local/etc/fake.env.file.not.there: No such file or directory

This seems fine to me and extra error handling probably isn't necessary unless someone else can think of another issue I'm overlooking.

[mat]

etc/rc.subr

962 ↗

(On Diff #39537)

[ -n "$_env_file" ] && [ -r "${_env_file}" ]

[feld]

Committing with the suggested change by mat.