branches: 1.3.2; 1.3.4;

Make expandm() return a const char * so we can do away with __UNCONST()
and more importantly, so that we don't accidentally return a value
that is a const char * in reality (pointer to read only string) as a
char *.

Provide an extra argument to store the returned pointer so we can use
the function directly as the return format (before assigning it to a
variable) to appease clang.

Use a utility function to expand %m correctly...