#49 improve non-fatal pp_cleanup() errors

[nasm64developer]

Currently pp_cleanup() generates a non-fatal
error if "defining" is not NULL.

This should be refined. In particular, the
error should hint at an unfinished %(I)MACRO
if "defining->name" is not NULL, but at an
unfinished %REP if it is NULL.

Furthermore there should be a check to see
if "ctsk" is NULL (before it's being cleaned
out). If it is not, then there should be a
nonfatal error about context "cstk->name"
still being defined, i.e. a missing %POP.

[Nickolay Yurchenko]

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Agreed.
What about non-empty context stack notification, it should be
a warning because a lot of programs may be written without
cleaning contexts.

Also, if we do so, I think, there should be a way to recognize
whether there is a context on the stack top or not. %(el)if(n)
ctx do not provide such functionality now - they only perform
a test of context name.

I want to propose such syntax:
%ifctx *

[nasm64developer]

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> [non-empty context stack at the end]

I don't really care whether it's a warning or
an error, as long as it does complain.

> [%(el)if(n)ctx should accept *]

I concur. I've just added that in my version.