Introduce free_and_strndup and use it in bus-message.c

Introduce free_and_strndup and use it in bus-message.c

v2: fix error in free_and_strndup()

When the orignal and copied message were the same, but shorter than specified
length l, memory read past the end of the buffer would be performed. A test
case is included: a string that had an embedded NUL ("q\0") is used to replace
"q".

v3: Fix one more bug in free_and_strndup and add tests.

v4: Some style fixed based on review, one more use of free_and_replace, and
make the tests more comprehensive.