[libc] Add getgrouplist entrypoint (#226958)

Add the getgrouplist entrypoint from <grp.h> (BSD extension), which
scans the group database to obtain the list of groups to which a user
belongs.

The base group passed by the caller is included unconditionally, and
supplementary groups for the user are gathered with duplicate group IDs
suppressed. A small-buffer-optimised container avoids heap allocations
for users belonging to up to 32 groups, falling back to dynamic
allocation when more groups are present. Note that this fallback does
not use AllocChecker because it relies on realloc to grow the buffer.
The lookup uses a scoped database stream to avoid disturbing concurrent
iteration.

Assisted-by: Automated tooling, human reviewed.
GitOrigin-RevId: 16036fdccdbcc14b5ae6afb54529733bb9741b0f