Catch "Select failed: Interrupted system call" entries from clamd.

When a select() call in clamd is interrupted, amavis logs the
following warning:

  ClamAV-clamd: Select failed: Interrupted system call at /usr/sbin/amavisd
  line 8472, <GEN16> line 296., retrying (1)

These are harmless in and of themselves, but we would like to classify
them as warnings. Currently they are unmatched. This commit adds a
pattern for them, moving them from the "Unmatched Entries" heading to
"Miscellaneous warnings" instead. Afterwards, the report looks like,

  24   Miscellaneous warnings --------------------------------
   2      ClamAV-clamd: Select failed: Interrupted system call