lei: support `daemon-env' for modifying long-lived env
While lei(1) socket connections can set environment variables
for its running context, it may not completely remove some of
them. The background daemon just inherits whatever env the
client spawning it had. This command ensures the persistent env
can be modified as needed.
Similar to env(1), this supports "-u", "-" (--clear), and
"-0"/"-z" switches. It may be useful to unset or change
or even completely clear the environment independently
of what a socket client feeds us.
"-i" is omitted since "--ignore-environment" seems like a bad
name for a persistent daemon as opposed to a one-shot command.
"-" and --clear (like clearenv(3)) will completely clobber
the environment.
"Lonesome dash" support is added to our option/help parsing
for the "-" shortcut to "--clear".
Getopt::Long doesn't seem to support specs like "clear|" or
"stdin|", but only "", so we do a little pre/post-processing
to merge the cases.