Mk/bsd.sites.mk: fall back to default WWW URLs for specific ports

This change has been accepted as part of review D36558. A follow-up
commit will apply the reminder of the patches in that review.

A separate commit is used since providing default values for certain
ports is logically different than support for multiple URLs in WWW.

This commit adds default WWW values if the following conditions are
met:

1. no value assigned to the WWW macro by the port

2. at least one of USE_GITHUB, USE_GITLAB, or USES=pear is used in the port

Further defaults for USES cases could be added at a later time.

The following values are used as default URLs:

USES=pear: https://pear.php.net/package/${PORTNAME}/
USE_GITHUB: https://github.com/${GH_ACCOUNT}/${GH_PROJECT}/
USE_GITLAB: https://gitlab.com/${GL_ACCOUNT}/${GL_PROJECT}/

If both a USES case and USE_GITHUB or USE_GITLAB apply, the default
URL of the USES case takes precedence.

Approved by: portmgr (tcberner)