fix scriptSig analysis in sign.cpp

Summary:
This is used for analyzing partially unsigned transactions, and exhibits
weird behaviour where it allows any opcodes (including CPU-intensive ones)
to be run, and then it analyses the aftermath.

Also, this is a misuse of MANDATORY_SCRIPT_VERIFY_FLAGS as it here we
shouldn't care about how invalid transactions are sometimes banned/not
banned in mempool acceptance. In Core they have STRICTENC here which
is pretty weird, since it doesn't really have any effect (invalidly
encoded signatures still get through since they are just pushed).

This changes the evaluation to only proceed if the script is properly
push-only, and uses neutral flags to build a stack out of the push
opcodes.

(In principle we could enforce MINIMALDATA, but, it doesn't hurt to
allow nonstandard pushes since all the pushes will get canonicalized by
PushAll after signing.)

Test Plan: ninja check-all

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D4907