When a well-formed range value, bounded by signed integers, is encountered
while processing a repeated option, enter LM_SIGNED_INTERVAL and return
the low bound.

Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Tested-by: Wanlong Gao <gaowanlong@cn.fujitsu.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>