Do not shadow variables (gcc set)

Summary:

• Do not shadow variables (gcc set)
• Check and enable -Wshadow by default.

This is a backport of core PR8808

Test Plan:

make check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1866