Add some more functions for -liberty and make the library WARNS=6 clean.

Add some more functions for -liberty and make the library WARNS=6 clean.

As replacement of the missing documentation:
- buildargv, freeargv and dupargv help with managing argv lists as NULL
  terminated lists of strings
- hex_p and hex_value help with the detection and evaluation of hex digests,
  they might be replaced by an inline version later
- concat and reconcat can be used to concat an arbitrary number of strings
- xatexit, xexit and xstrerror just call the corresponding normal functions,
  they already provide the GNU functionality
- xcalloc, xmalloc, xmemdup, xrealloc and xstrdup work as expected but
  terminate the program if the memory allocation can not be fullfilled