Fixes the caddy_site_activity plugin to properly support Caddy configurations with multiple server blocks, adds checks to prevent errors from undefined values

[mgc8]

The plugin was hardcoded to only look at srv0 in the Caddy configuration:

my $caddy_internal_names = $config->{apps}{http}{servers}{srv0}{logs}{logger_names};

This caused the plugin to fail or provide incomplete data when:

• Caddy configurations use multiple named server blocks (e.g., srv0, srv1, etc.)
• Server blocks are named differently than srv0
• The configuration has additional servers for different ports or protocols

Changes

1. Multi-server support

• Iterate through all servers: Now loops through all entries in $config->{apps}{http}{servers} to build a complete mapping of internal names to hostnames
• Applied to both config and fetch modes for consistency

2. Hostname filtering

• Modified count_json_log_hits(): Changed the function to accept $hostname instead of unused $statuses parameter
• Filter log entries: Only count hits for log entries that match the specific hostname we're processing => this ensures accurate hit counts when multiple sites share the same log file or when processing multiple sites

3. Check for undefined values

• Added defined checks before comparing:
  • $$mapping{$_} in internal_name_to_site_name()
  • $logfile->{writer}{output} and $logfile->{encoder}{format} before string comparisons
  • Log entry fields (status, request, request{host}) before accessing them
• Added explicit close(LOG_FILE) call
• Skip unknown sites in fetch mode: Removed the code that printed "U" (unknown) values for unidentified sites, now just skips them like in config mode

Tested and confirmed to work under Munin 2.0.73 with no errors nor warnings

Fixes #1