[libc-eglibc] Rewrite part of the code

Rewrite part of the code to better match the rest.
Most notably, rewrite handling of:
if [ ... ] && [ ... ]
to:
if [ ... -a ... ]

This has the positive side effect of calling "[" only once, although
"[" is probably a shell built-in.

 menu "Debug facilities"

 source config.gen/debug.in

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