Function: getenv-internal

Get the value of environment variable VARIABLE.
VARIABLE should be a string. Value is nil if VARIABLE is undefined in
the environment. Otherwise, value is a string.

This function searches `process-environment' for VARIABLE.

If optional parameter ENV is a list, then search this list instead of
`process-environment', and return t when encountering a negative entry
(an entry for a variable with no value). (fn VARIABLE &optional ENV)