[Toybox] pidof cleanups.

[Rob Landley]

At the design level, the new library function should really pass in a
pid_t not a char *, so I changed that.  Also, the large amount of string
manipulation you're doing boils down to a single call to snprintf().

You don't need to zero out global variables, they start prezeroed unless
set to something else by the argument parsing logic.

Sticking "<1" at the start of the option string makes it complain if you
supply less than one argument.

If you return from a command_main() toybox will exit with the value
defined in toys.exitval.  (It defaults to zero. Note that if you set it
to a value _other_ than zero, error_exit() and the various functions
that call it (perror_exit() and anything starting with x) will use that
as their exit code; if toys.exitval is 0 they'll exit with 1.

This means I can just use that global variable, initialize it to 1 and
then set it to 0 if we find any processes, and thus don't have to define
a new global variable just for this program.  (Probably doesn't save any
space in the binary, but saves several lines in the source code.)

Rob