Description of problem:

In case someone is editing a file using *vim*, e.g. `vim /etc/dnf/vars/releasever` and keeps the editor open, or editor died without having deleted the `.releasever.swp` file), DNF dies continuously when trying to decode the content:

-------- 8< ---------------- 8< ---------------- 8< ---------------- 8< --------
# dnf install net-snmp
Config error: 'utf-8' codec can't decode byte 0x8d in position 16: invalid start byte
-------- 8< ---------------- 8< ---------------- 8< ---------------- 8< --------

The reason for this is DNF reads and processes the swp file, as seen with strace:
-------- 8< ---------------- 8< ---------------- 8< ---------------- 8< --------
 :
721625 11:53:32.319506 read(6</etc/dnf/vars/.releasever.swp>, "b0VIM 8.0\0\0\0\0\20\0\0\213,\304a\234\36020V\16\0\0root\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0xpointwebsrv-new\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\
 :
-------- 8< ---------------- 8< ---------------- 8< ---------------- 8< --------

Apparently all files in /etc/dnf/vars are processed, even if the filename doesn't look like a DNF variable.
We need to harden this to avoid either opening non-expected filenames, or continue the processing without dying.

Version-Release number of selected component (if applicable):

dnf-4.7.0-8.el8.noarch

How reproducible:

Always

Steps to Reproduce:
1. Create/open /etc/dnf/vars/releasever using *vim*

  # vim /etc/dnf/vars/releasever

  Write "8" to it, save and keep the editor open

2. Try installing a package from another terminal

  # dnf install net-snmp

Actual results:

Config error: 'utf-8' codec can't decode byte 0x8d in position 16: invalid start byte

Expected results:

DNF continuing