kdump-lib: Introduce a help function _crashkernel_add()
    
    Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=2160676
    Upstream: Fedora rawhide
    Conflict: Drop shellspec test case
    
    commit 51efbcf83e26a69fb56466a580b50e5d4867509f
    Author: Pingfan Liu <piliu@redhat.com>
    Date:   Tue Jun 13 17:43:20 2023 +0800
    
        kdump-lib: Introduce a help function _crashkernel_add()
    
        This help function can manipulate the crashkernel cmdline by adding an
        number for each item. Also a basic test case for _crashkernel_add() is
        provided in this patch.
    
        Credit to Philipp, who contributes the original code.
    
        Signed-off-by: Pingfan Liu <piliu@redhat.com>
        Reviewed-by: Coiby Xu <coxu@redhat.com>
        Reviewed-by: Philipp Rudo <prudo@redhat.com>
    
    Signed-off-by: Pingfan Liu <piliu@redhat.com>