Add a META robots noindex/nofollow tag to SquirrelMail generated output.

Add a META robots noindex/nofollow tag to SquirrelMail generated output.
Major search engines will not index squirrelmail-pages. This serves the
following points:
- The search engines' cache/index does not get polluted with useless data;
  indexing a webmail client is not of interest. We should advise search
  engines that they skip us.
- Will not make a squirrelmail install findable through google. This has
  an advantage when someone is looking for vulnerable webmail systems to
  exploit (i.e.: search for "SquirrelMail" in google, and try every system
  you find with a known exploit).
A robots.txt is not a solution because that has to be in the root of a
document tree, which squirrelmail does not have to be.

git-svn-id: https://svn.code.sf.net/p/squirrelmail/code/trunk/squirrelmail@9187 7612ce4b-ef26-0410-bec9-ea0150e637f0