Add runtime detection and bypass toggle for php-snmp

[somethingwithproof]

When php-snmp is installed, Cacti uses it for SNMP operations instead of the net-snmp CLI tools. This causes problems in several scenarios:

• SNMPv3 polling fails silently (see Note about php_snmp and snmp v3  #3818)
• On FreeBSD, SO version bumps in net-snmp break php-snmp linkage, causing data retrieval issues (see Interface statistics Data Query does not list ifNames #6644)
• The <output_format> tag in data query XML templates behaves differently under php-snmp vs net-snmp CLI

The README recommends removing php-snmp, but this isn't always practical — FreeBSD ports pull it in as a Cacti dependency, and other applications on the same host may require it.

Proposed changes:

1. Add a Settings > Poller toggle to force net-snmp CLI mode even when php-snmp is present
2. Add a notice on the System Utilities page when php-snmp is detected
3. Log a NOTICE-level message at poller startup when php-snmp is loaded

Ref #6644, #3818

/cc @TheWitness @xmacan

[somethingwithproof]

Upstream context: php/php-src#20752 tracks a related ext/snmp bug where hostname resolution fails on IPv4-only systems. A partial fix is at php/php-src#21000 — I posted a comment there with a suggested fallback for the remaining failure case.

[somethingwithproof]

Additional upstream context: php/php-src#21132 was initially reported as a PHP 8.5.1 regression (second SNMP GET fails), but the reporter traced it to net-snmp 5.9.5.2 — same root cause as php/php-src#20752. Pinning to net-snmp 5.9.4.pre2 works around both issues. This reinforces the case for a CLI-mode bypass toggle that avoids ext/snmp entirely.

[somethingwithproof]

Upstream progress: php/php-src@687ac23 landed a fix for IPv4-only systems where getaddrinfo() returns ::ffff: mapped addresses that php-snmp formats as udp6:[addr] — Net-SNMP can't connect to those. The fix prefers AF_INET results when IPv6 isn't explicitly requested.

This addresses one failure mode but doesn't cover the SNMPv3 silent failures (#3818) or FreeBSD SO version linkage breaks (#6644), so the bypass toggle is still needed.

[somethingwithproof]

Tracked in #7034 (polling engine roadmap).