What would be a a linguistic term for those nouns ending with -ing?

They could be one of two things, Gerunds or Deverbal Nouns.

The wikipedia article on Gerunds has a decent section on the nominal and verbal properties of gerunds.

Nominal characteristics

1. The gerund can perform the function of subject, object and predicative
2. The gerund can be preceded by a preposition
3. Like a noun the gerund can be modified by a noun in the possessive case, a possessive adjective, or an adjective

Verbal characteristics

1. The gerund of transitive verbs can take a direct object
2. The gerund can be modified by an adverb
3. The gerund has the distinctions of aspect and voice

EDIT

Based on feedback I'll point out a specific section of that Wikipedia article.

Not all nouns that are identical in form to the present participle are gerunds. The formal distinction is that a gerund is a verbal noun – a noun derived from a verb that retains verb characteristics, that functions simultaneously as a noun and a verb, while other nouns in the form of the present participle (ending in -ing) are deverbal nouns, which function as common nouns, not as verbs at all.

So from your examples, the grammatical term you might be looking for is Deverbal Nouns. Only a subset of deverbal nouns end in -ing, however.

I differ somewhat with @ghoppe on this issue. A gerund is still a verb that is being used as a noun with the addition of -ing. But words like "scaffold" are not verbs at all (except as English permits nouns to be used as verbs). So that fails the gerund "sniff" test for me.

If we look at Webster's 3rd New International Dictionary for -ing, we find this tidbit:

4: something connected with, consisting of, or used in making (a specified thing) <scaffolding>, <sacking> <shirting> — in nouns, esp. collectives formed from nouns

5: something related to (a specified concept) <offing> in nouns formed from parts of speech other than verbs and nouns;

So while the -ing ending originated as a participial ending and later becoming a gerundial suffix, calling all words ending in -ing gerunds is a bit overstating the case. The fact that nouns are formed from other parts of speech by taking -ing endings would seem to eliminate gerund as a catch-all name for this kind of word.