MINOR: chunks: add chunk_strcat() and chunk_newstr()

MINOR: chunks: add chunk_strcat() and chunk_newstr()

These two new functions will make it easier to manipulate small strings
from within functions, because at many places, multiple short strings
are needed which do not deserve a malloc() nor a free(), and alloca()
is often discouraged. Since we already have trash chunks, it's convenient
to be able to allocate substrings from a chunk and use them later since
our functions already perform all the length checks. chunk_newstr() adds
a trailing zero at the end of a chunk and returns the pointer to the next
character, which can be used as an independant string. chunk_strcat()
does what it says.