Drop direct dependencies to globals in is_blacklisted().

git-svn-id: http://textpattern.googlecode.com/svn/development/4.x@5399 2fea6a4d-a838-0410-917b-93a53c48e9d2
is_blacklisted()'s $checks takes an array.

Added an additional support for strings.

git-svn-id: http://textpattern.googlecode.com/svn/development/4.x@5400 2fea6a4d-a838-0410-917b-93a53c48e9d2
Dropped direct references to $prefs global from txpMail().

git-svn-id: http://textpattern.googlecode.com/svn/development/4.x@5401 2fea6a4d-a838-0410-917b-93a53c48e9d2
Dropped references to $prefs from encode_mailheader().

git-svn-id: http://textpattern.googlecode.com/svn/development/4.x@5402 2fea6a4d-a838-0410-917b-93a53c48e9d2