title: Delete column with NAs in first row
body: <p>If I have a dataframe like so</p>
  
  <pre><code>a &lt;- c(NA,1,2,NA,4)
  b &lt;- c(6,7,8,9,10)
  c &lt;- c(NA,12,13,14,15)
  d &lt;- c(16,NA,18,NA,20)
  df &lt;- data.frame(a,b,c,d)
  </code></pre>
  
  <p>How can I delete columns "a" and "c" by asking R to delete those columns that contain an NA in the first row?</p>
  
  <p>My actual dataset is much bigger, and this is only by way of a reproducible example.</p>
  
  <p>Please note that this isn't the same as asking to delete columns with any NAs in it.  My columns may have other NA values in it.  I'm looking to delete just the ones with an NA in the first row.</p>