MINOR: chunk: make chunk_initstr() take a const string

chunk_initstr() prepares a read-only chunk from a string of
fixed length. Thus it must be prepared to accept a read-only
string on the input, otherwise the caller has to force-cast
some const char* and that's not a good idea.
(cherry picked from commit 70af633ebe8627fdf6b448f96c0e457adc93b1d1)

[wt: not strictly needed but backported to ensure that any future patch
 relying on it works as expected]
(cherry picked from commit 7d16f278c6f2332439c7ddfbd939fbc48ddb5450)
1 file changed