estack.eclass: Properly restore shopt options.

estack.eclass: Properly restore shopt options.

Calling "eshopts_push; eshopts_pop" makes Portage report a QA issue:
* QA Notice: Global shell options changed and were not restored while calling 'src_prepare'

This is caused by some side effect in bash, by which disabling
the "posix" option (even if it was already disabled before) in a
non-interactive shell also disables the "expand_aliases" option.
Work around the problem by always saving and restoring both "set -o"
and "shopt" option sets.

Also fix "estack_push -s" which should not execute shopt when called
without further parameters.

Closes: https://bugs.gentoo.org/662586
Signed-off-by: Ulrich Müller <ulm@gentoo.org>