lib/argv: introduce setup_env and getenv

Provide a function that imports an environ (a list of key=value
environment variables). The list may be delimited by either
'\0' or '\n'. If the list is delimited by '\n', then we assume
it's user input and do additional sanity checking, as well as
allow variables to be commented out with '#'. We also provide
getenv() to lookup the variables.

Signed-off-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>