sysutils/fluent-bit: Improve RC script and fix issues

• Document 'fluent_bit_group' variable
• Install PID file with mode 0600 instead of 0755
• Reuse ${name}
• Pass '-t {name}' to daemon(8)
• Remove self-created PID file

PR: 292782
MFH: 2026Q1
Approved by: girgen (maintainer)

(cherry picked from commit efa59bb21618021d2726aed339bf448aa055fc4e)