libbb: make '--help' handling more consistent

Running an applet with '--help' as its only argument is treated
as a special case.  If additional arguments follow '--help' the
behaviour is inconsistent:

- applets which call single_argv() print help and do nothing else;

- applets which call getopt() report "unrecognized option '--help'"
  and print help anyway;

- expr says "expr: syntax error" and doesn't print help;

- printenv silently ignores '--help', prints any other variables
  and doesn't print help;

- realpath says "--help: No such file or directory", prints the path
  of any other files and doesn't print help.

If the first argument is '--help' ignore any other arguments and print
help.  This is more consistent and most likely what the user wanted.

See also commit 6bdfbc4cb (libbb: fix '--help' handling in
FEATURE_SH_NOFORK=y).

function                                             old     new   delta
show_usage_if_dash_dash_help                          75      69      -6
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(add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-6)               Total: -6 bytes

Signed-off-by: Ron Yorston <rmy@pobox.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
1 file changed