untangle them:
Rewrite u_signal_names() into get_signum() and get_signame(), plus trim the
signal list to that required by posix (they can specify the numbers for
the rest if they really need them). (This is preparatory cleanup for adding
a timeout applet like Roberto Foglietta wants.)
Export the itoa (added due to Denis Vlasenko, although it's not quite his
preferred implementation) from xfuncs.c so it's actually used, and remove
several other redundant implementations of itoa and utoa() in the tree.
