Harden agent log file creation in the shared log directory

[teta2k]

Summary

The agent writes its logs to /var/log/aikido-<version>/, which is world-writable so PHP processes running as the web server user can log there too. Log file names are predictable, so another local user could create a file or symlink under the name before the agent opens it.

• Log file creation: open agent log files with O_CREATE|O_EXCL, so an existing file or symlink (including a dangling one) is never followed or reused. If the name is taken, the agent logs to stdout as it already does when the open fails. New log files are 0644 instead of 0666.
• Unique per-server log names: per-server log files now include the PHP server PID (aikido-agent-<time>-<agent pid>-<token tail>-<server pid>.log). One token can register several servers, and two of them registering in the same second used to share one file name; they opened the same file at offset 0 and overwrote each other's lines. With O_EXCL the second one would have fallen back to stdout, so the names now stay unique instead.
• Sticky bit: the apk and rpm install scripts now create /var/log/aikido-* and /run/aikido-* as 1777 instead of 0777, so users can't delete or replace each other's files there. On most distros this also enables the kernel's protected_symlinks / protected_regular checks for these directories.
• Log line escaping: CR/LF in log messages are escaped, so logged request data (domains, routes, ...) can't add extra log lines. Trailing newlines are trimmed first, so existing ...\n messages don't print a literal \n.

Notes

• With the sticky bit, a non-root agent can no longer remove a stale socket in /run/aikido-* left behind by a root agent.

Testing

• New lib/agent/log/log_test.go: planted symlink, dangling symlink and pre-existing file are all refused and left untouched; new file is not group/world-writable; newlines are escaped; trailing newlines are trimmed.
• Agent builds; go vet on log and server_utils is clean; go test ./... in lib/agent passes on Linux.