(V9_ECHO): Remove; always enabled.

(V9_ECHO): Remove; always enabled.
(DEFAULT_ECHO_TO_XPG): Renamed from V9_DEFAULT, so that
we use the same naming convention as bash.  Now an enum,
not a macro.
(usage): Reword to mention -e/-E more accurately.
Mention \0NNN (the POSIX syntax) rather than \NNN (nonstandard).
(hextobin): New function.
(main): Use bool rather than int for local vars when appropriate.
Do not allow options if POSIXLY_CORRECT, unless we are using
BSD semantics and the first argument is "-n".
Don't pass unnecessary extra arg to parse_long_options.
do_v9 now defaults to DEFAULT_ECHO_TO_XPG, not to allow_options.
Do not look for options if !allow_options.
Use size_t rather than int when appropriate.
Open-code option test rather than using strrchr.
Use faster test for "-".
Avoid redundant argc test.
Add support for \x, for Bash compatibility.
Use e.g. '\a' rather than '\007', for portability to EBCDIC hosts.
When '\c' is encountered, stop printing immediately, as POSIX
requires.
Add support for \xhh syntax.
Add support for \0ooo syntax; POSIX requires this.