first attempt at issue #2756, input-limited putnstr functions

[zhiayang]

Totally fell off the earth there, put the project I was doing on hiatus oops.

This is a first attempt implementing #2756; ofc the names are not final, I've just put another n to get putnstrn and egcn etc.

Some thoughts:

1. There's other functions that end up calling ncplane_putegc, so we should probably also see if we want similar things for that family of functions.
2. ncplane_putegc actually already takes a size_t* as an out-parameter; if we didn't care about API breakage we could specify that it is also an input parameter that specifies the length of the input string. not the best idea though.
3. small amount of code duplication, not sure how to fix that

[zhiayang]

@dankamongmen would appreciate a look at this if you missed it (: