Syscalls str filter

[lacraig2]

This pull request introduces support for string-based filtering in syscall hooks, adds robust string comparison helpers, and ensures proper memory management for dynamically allocated filter patterns. Additionally, it includes minor type and variable declaration improvements in the portal subsystem for clarity and correctness.

Syscall hook filtering improvements:

• Added new string-based filter types to enum value_filter_type, enabling exact match, contains, startswith, and endswith comparisons for syscall argument and return value filtering.
• Extended the struct value_filter to include a pattern pointer and its length for string comparisons.
• Updated value_matches_filter to handle the new string filter types, invoking the appropriate helper for each.

String comparison helpers:

• Introduced efficient, chunked string comparison helpers (check_str_exact, check_str_startswith, check_str_endswith, check_str_contains) to safely compare user-space strings from kernel space, minimizing stack usage and handling large strings.

Memory management:

• Ensured that dynamically allocated string patterns in argument and return value filters are properly freed during syscall hook unregistration to prevent memory leaks.

Portal subsystem improvements:

• Changed variable declarations in portal_get_vma_name to avoid shadowing and improve clarity; switched some types to const char * where appropriate. [1] [2] [3] [4]
• Updated the type of copy_size in handle_op_copy_buf_guest to uint64_t for correctness and used min_t for type safety.

Other minor changes:

• Included <linux/uaccess.h> in syscalls_hc.h to support user-space memory access in the new helpers.
• Added a missing variable declaration (int i) in do_unregister_syscall_hook.