useradd: make the option specstr passed to getopt_long extensible

useradd: make the option specstr passed to getopt_long extensible

It was hard to extend the option specification string passed to
getopt_long as the third argument.

The origian code had a branch with WITH_SELINUX ifdef condition. If
one wants to add one more option char with another ifdef condition
like ENABLE_SUBIDS to the spec, the one must enumerate the specs for
all combinations of the conditions:

*  WITH_SELINUX &&  ENABLE_SUBIDS
*  WITH_SELINUX && !ENABLE_SUBIDS
* !WITH_SELINUX &&  ENABLE_SUBIDS
* !WITH_SELINUX && !ENABLE_SUBIDS

With this change, you can append an option char to the spec.

Signed-off-by: Masatake YAMATO <yamato@redhat.com>