security/openvpn-devel: upgrade port to git commit 0fb5a00549 (2.7_beta2, 2025-09-25)

This commit brings the port to "openvpn 2.7_beta2".

Notable changes beta1 -> beta2 (relevant for FreeBSD) are:

• even more of signed/unsigned related warnings have been fixed
• #pragmas have been added to all to-be-fixed source files, so we can now always enable -Wconversion to see if new code brings new warnings (and the CI infra builds with -Werror)
• add proper input sanitation to DNS strings to prevent an attack coming from a trusted-but-malicous OpenVPN server (CVE: 2025-10680, affects unixoid systems with --dns-updown scripts and windows using the built-in powershell call)
• Switch test_ssl certificate from RSA 2048 to secp384r1 (so "make check" runs with OpenSSL set to @SECLEVEL=3)
• clean up MI prefix handling
• replace all assert() calls with OpenVPN ASSERT()

PR: 289838
Security: e5cf9f44-9a64-11f0-8241-93c889bb8de1
Security: CVE-2025-10680
MFH: 2025Q3