add --vmrss-adjust-percent option for dynamic VMRSS adjustment

This commit adds a new --vmrss-adjust-percent command-line option that allows users to set the VMRSS adjustment value as a percentage of total memory instead of using the fixed 3 GiB default. Key changes: - Add --vmrss-adjust-percent option to set VMRSS adjustment as percentage of total memory for --prefer/--avoid processes when using --sort-by-rss - Store vmrss_adjust_percent and total_memory_kib in poll_loop_args_t - Calculate dynamic VMRSS adjustments based on percentage if specified - Maintain backward compatibility with fixed 3 GiB default values - Update documentation in MANPAGE.md and README.md - Add validation for percentage value (0-100)