src/libudev/strv.{c,h}: bring in line with upstream

In doing so, we better alined util.{c,h} by movig the definitions
of freep, fclosep, closep, closedirp, umaskp from util.c to util.h
as static inline.  We also add strv_env_get to env-util.{c,h} which
we forgot to do earlier.

Signed-off-by: Anthony G. Basile <blueness@gentoo.org>