Using no proxy option to skip wget and curl proxy settings in config

When end users specify proxy settings in config file for wget /etc/wgetrc:
http_proxy = http://...
or for curl ${HOME}/.curlrc:
proxy = http://...

Using `http_proxy="" wget' can not skip the proxy setting in the
config files, also it can skip proxy settings in env viriables.

In order to skip proxy setting in both env and config file, we pass
--no-proxy option for wget, and --noproxy '*' for curl.

Fixes bug #1224836

Change-Id: I2b25aeca9edf2ce4525fb1db325e5e24c18b4d55
set errexit and xtrace in helper scripts

stack.sh invokes some helper scripts as separate processes, rather than
by source'ing them.  As with stack.sh itself, abort immediately on the
first error, so that errors don't compound and result in confusing error
messages.  If one of these helper scripts aborts, stack.sh itself will
also abort in the usual manner.

Due to the change in behaviour, tweak some mv invocations to ensure that
they don't trigger false failures.

As with stack.sh itself, also enable xtrace so we can see exactly what's
happening.  In particular this allows us to see the cause of any
premature termination due to a command failing whilst errexit is
enabled.

Change-Id: I7a55784c31e5395e29ab9bbe2bb112b83b9be693
Merge "set errexit and xtrace in helper scripts"
Merge "Using no proxy option to skip wget and curl proxy settings in config"