MEDIUM: init: allow the redefinition of argv[] parsing function

MEDIUM: init: allow the redefinition of argv[] parsing function

This patches allows the argv[] parsing function to be redefined from
others C modules. This is done extracting the function which really
parse the argv[] array to implement haproxy_init_args(). This function
is declared as a weak symbol which may be overloaded by others C module.

Same thing for copy_argv() which checks/cleanup/modifies the argv array.
One may want this function to be redefined. This is the case when other
C modules do not handle the same command line option. Copying such
argv[] would lead to conflicts with the original haproxy argv[] during
the copy.