fix #3391: improve mismatched fingerprint handling

fix #3391: improve mismatched fingerprint handling

if the expected fingerprint and the one returned by the server don't
match, print a warning and allow confirmation and proceeding if running
interactive.

previous:

$ proxmox-backup-client ...
Error: error trying to connect: error:1416F086:SSL routines:tls_process_server_certificate:certificate verify failed:../ssl/statem/statem_clnt.c:1915:

new:

$ proxmox-backup-client ...
WARNING: certificate fingerprint does not match expected fingerprint!
expected:    ac:cb:6a:bc:d6:b7:b4:77:3e:17:05:d6:b6:29:dd:1f:05:9c:2b:3a:df:84:3b:4d:f9:06:2c:be:da:06:52:12
fingerprint: ab:cb:6a:bc:d6:b7:b4:77:3e:17:05:d6:b6:29:dd:1f:05:9c:2b:3a:df:84:3b:4d:f9:06:2c:be:da:06:52:12
Are you sure you want to continue connecting? (y/n): n
Error: error trying to connect: error:1416F086:SSL routines:tls_process_server_certificate:certificate verify failed:../ssl/statem/statem_clnt.c:1915:

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>