(xreadlink): Omit size_t* arg. All uses changed.

(xreadlink): Omit size_t* arg.  All uses changed.
Use ssize_t, not int, to store result of readlink.
Check for ssize_t overflow as well as size_t overflow,
as POSIX says the result of readlink is implementation-defined
when ssize_t overflows.
Remove unnecessary cast to char*.
Use free+malloc instead of realloc, as the storage doesn't need
to be preserved and it's clearer and can be more efficient that way.
(SIZE_MAX, SSIZE_MAX): New macros, if <limits.h> doesn't declare.