Fix unsigned comparison warnings in chown()

Fix unsigned comparison warnings in chown()

Original warnings:
src/filefunc.cpp:756:12: error: comparison of unsigned expression in '< 0' is always false [-Werror=type-limits]
src/filefunc.cpp:758:12: error: comparison of unsigned expression in '< 0' is always false [-Werror=type-limits]
src/filefunc.cpp:759:12: error: comparison of unsigned expression in '< 0' is always false [-Werror=type-limits]

The uid_t and gid_t types are unsigned, so comparisons like 'uid < 0' are always false.
Changed to use static_cast<uid_t>(-1) and static_cast<gid_t>(-1) to check for sentinel values.