debuginfod-doc: PR27950 - Remove redanduncies in man page.

debuginfod-doc: PR27950 - Remove redanduncies in man page.

Create a new file, debuginfod-client-config.7, that holds all
environment variables and cache control files related info. Get rid of
repetitive definitions in three other files, instead, those files will
include the content of new file. Any future modification related to
environment variables and cache files will only require changes in one
file.

Signed-off-by: Alice Zhang <alizhang@redhat.com>
Signed-off-by: Frank Ch. Eigler <fche@redhat.com>