build: avoid a warning form gcc's new -Wlogical-op

build: avoid a warning form gcc's new -Wlogical-op

Without this change, very recent gcc (e.g., version 6.0.0 20150509)
would print the following when configured with --enable-gcc-warnings:

  src/copy.c:165:30: error: logical 'or' of equal expressions \
    [-Werror=logical-op]
    && (errno == EOPNOTSUPP || errno == ENOTSUP || errno == ENOSYS))
                           ^
* src/system.h (is_ENOTSUP): New function.
* src/copy.c (punch_hole): Use it.
* src/ls.c (errno_unsupported, gobble_file): Use it.