Added two new parameters for handling of multi-term varlistentry

Added two new parameters for handling of multi-term varlistentry
elements:

variablelist.term.break.after:
  When the variablelist.term.break.after is non-zero, it will
  generate a line break after each term multi-term varlistentry.

variablelist.term.separator:
  When a varlistentry contains multiple term elements, the string
  specified in the value of the variablelist.term.separator
  parameter is placed after each term except the last. The default
  is ", " (a comma followed by a space). To suppress rendering of
  the separator, set the value of variablelist.term.separator to
  the empty string ("").

These parameters are primarily intended to be useful if you have
multi-term varlistentries that have long terms.

Closes #1306676. Thanks to Sam Steingold for providing an example
"lots of long terms" doc that demonstrated the value of having
these options.

Also, added normalize-space() call to processing of each term.

This change affects all output formats (HTML, PDF, manpages). The
default behavior should pretty much remain the same as before, but
it is possible (as always) that the change may introduce some new
bugginess.