musl: add getopt support for non-option arguments

musl libc doesn't support the GNU getopt extension to parse non-option
arguments when the optstring starts with '-'.

This extension is used by some utilities, notably iptables, that
currently return with errors even with perfectly valid invocations.

The patch adds the code needed by getopt.c and getopt_long.c to
implement that extension.

Signed-off-by: Gianluca Anzolin <gianluca@sottospazio.it>

SVN-Revision: 43309