reboot: Implement -e from nextboot

[imp]

Summary

Allow -e foo=bar to add loader environment variables to nextboot.conf.

Sponsored by: Netflix

[kib]

sbin/reboot/reboot.c

119

150

Do we need the precise calculation of the dst len? IMO always allocating 3 * slen + 1 would be good enough.

Same about the special case of "XXX" string, but I am not sure there. Why having a different behavior WRT quoting comparing with other cases?

201

[imp]

Update based on kib's quoting feedback. I'm reverting to just adding quotes, if
they aren't already present like the old code. I've added a note to the man page
about this option and the need for care.

[imp]

refactor adding an env var.

[kib]

sbin/reboot/reboot.c

161

185

[imp]

update per review