amavis-logwatch: support forthcoming amavis-2.12.0 "starting" format.

When amavis-2.12.0 starts or reloads, it outputs its name as "amavis"
instead of the "amavisd-new" that previous versions used. This commit
adds support for the name "amavis". It also allows the "starting" line
to contain "(warm)", with the "(warm)" being ignored so that the name
of the amavis daemon is captured properly.

This fixes missed lines of the form,

  Aug  6 00:00:54 mx1 amavis[10199]: starting. (warm) /usr/sbin/amavisd at
  mx1.example.com amavis-2.12.0 (20190725), Unicode aware, LANG="C.utf8"

which is very likely to be the final format used by amavis-2.12.0.