buffer_fromsa - initialize buffer structure from stralloc
,const stralloc* sa
buffer_fromsa makes a virtual read buffer from a stralloc. The buffer reading
functions will be able to read until the end of the data in the stralloc, then
signal EOF. The buffer does not have its own buffer space, it reuses the one
in the stralloc.
Do not touch the stralloc until you are done with the buffer! In particular, do
not call stralloc_free on it!
Reading data from the buffer will not change the stralloc.
buffer_tosa(3), buffer_init(3), buffer(3), buffer_frombuf(3),