maint: prefer issymlink to readlink with a small buffer

maint: prefer issymlink to readlink with a small buffer

* bootstrap.conf (gnulib_modules): Add issymlink and issymlinkat.
* src/copy.c: Include issymlink.h.
(copy_reg): Use issymlink instead of readlinkat.
* src/rmdir.c: Include issymlink.h.
(main): Use issymlink instead of readlink.
* src/tail.c: Include issymlink.h.
(recheck, any_symlinks): Use issymlink instead of readlink.
* src/test.c: Include issymlink.h.
(unary_operator): Use issymlink instead of readlink.