Rewrite from scratch. Support everything

Rewrite from scratch.  Support everything
required by POSIX 1003.1-2001; when this conflicts with Bash,
stick with POSIX.  The conflicts are kill -l output format,
and lower case signal names preceded by `-' (e.g., "kill -hup"
is no longer supported).  Remove -L or --long-list option.
Add -t or --table option.  Rename --sigspec to --signal;
remove --signum and do not advertise obsolescent option -n.
Use str2sig and str2sig to convert between signal names and
numbers.