rc.subr: Support loading environmental variables from a file

The current support for setting environment via foo_env="" in rc.conf is
not scalable and does not handle envs with spaces in the value. It seems
a common pattern for some newer software is to skip configuration files
altogether and rely on the env. This is well supported in systemd unit
files and may be the inspiration for this trend.

MFH: 1 week
Differential Revision: https://reviews.freebsd.org/D14453