Currently we replace the environment of subprocesses with our own,
primarily only consisting of METER_* entries. Besides clearing
some very useful system dependent variables (like PATH, LDPATH, etc.)
with also prevent the user from passing environment variables to
invoked commands.
This patch will now copy the current environment and extends it with
the variables we want to set.
Signed-off-by: Christian Speich <christian@spei.ch>