util: Use g_malloc/g_free in envlist.c

Change malloc/strdup/free to g_malloc/g_strdup/g_free in
util/envlist.c.

Remove NULL checks for pointers returned from g_malloc and g_strdup
as they exit in case of failure. Also, update calls to envlist_create
to reflect this.

Free array and array contents returned by envlist_to_environ using
g_free in bsd-user/main.c and linux-user/main.c.

Update comments to reflect change in semantics.

Signed-off-by: Saurav Sachidanand <sauravsachidanand@gmail.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
