[th/str-buf-stack-allocated] glib-aux: add support for starting with stack-allocated buffer in NMStrBuf

Allow to initialize NMStrBuf with an externally allocated array. Usually that would be a stack buffer. If the NMStrBuf grows beyond the size of that initial buffer, then it would switch using malloc.

The idea is to support the common case where the result is small enough to fit on the stack.

I always wanted to do such optimization because the main purpose of NMStrBuf is to put it on the stack and ad-hoc construct a string. I just figured, it would be complicate the implementation and add a runtime overhead. Now I think the implementation not complicated and the usage convenient.