For complete putenv()-like behaviour.

Further investigation on why putenv() caused EFAULTs on execve() and
segfaults on reload:

putenv(3):

   "The string pointed to by string becomes part of the environment,
    so altering the string changes the environment."

setenv(3):

   "This function makes copies of the strings pointed to by name and value
    (by contrast with putenv(3))."
Signed-off-by: Stefan Knoblich <s.knoblich@axsentis.de>