security/clamav-lts: Add new port

• Add Long Term Support release of ClamAV
• Add CONFLICTS_INSTALL to security/clamav
• Add entry to UPDATING that explains
  • Upstream changed their end-of-life policy and LTS release is introduced
  • How to switch from regular release (security/clamav) to LTS release (security/clamav-lts)

Note about setting PORTEPOCH

In general new port should not set PORTEPOCH. But in this case it
should be set with following reason.

1. Recently upstream changed their end-of-life policy as following.
   • Regular feature release (= 0.xyz.0 release) is released more frequently, and life time of each feature release is shortened to about 4 months.
   • To compensate for the short lifetime of regular feature release, Long Term Support feature release is introduced and its life time is about 3 years.
2. First LTS starts with version 0.103.3 and it is same as current version of security/clamav.
3. Because of short lifetime of regular feature release, it is probable that not a few users of security/clamav want to switch to security/clamav-lts after the latter is connected (and the former is updated to 0.104.0).
4. For such users the entry is added to UPDATING about how to switch from security/clamav to security/clamav-lts. And binary package user is suggested to execute pkg set -o security/clamav:security/clamav-lts. This command changes the origin of already installed clamav packages. So user can switch to LTS version without reinstalling.
5. But if PORTEPOCH isn't set in security/clamav-lts, binary package user who executed above command will notice that version of installed package (0.103.3,1) succeeds to current version of security/clamav-lts (0.103.3). The situation doesn't change after new patch release (0.103.4 for example) is released and it causes the problem that installed package isn't properly upgraded with pkg upgrade.
6. So PORTEPOCH is set in security/clamav-lts to prevent binary package user from suffering from such problem.