Syscall migrations of stdio and unistd (#196403)
Added ErrorOr-returning syscall wrappers for access, chdir, dup, dup2,
dup3, faccessat, fchdir, fsync, lseek, readlink, readlinkat, rename,
rmdir, and unlinkat.
Migrated the Linux entrypoint implementations in src/unistd/linux/ and
src/stdio/linux/rename.cpp to use them.
Replaced internal::lseekimpl() with linux_syscalls::lseek() in the
File infrastructure and deleted the now-unused lseekImpl.h.
Assisted-by: Automated tooling, human reviewed.
