Add checks for ChanNums with leading _ character.

This was from the Beta user's group and Brian Murrell. He had an
old recording that contained a chanid that found a channum _59-27.
That causes NPEs as it didn't convert to a float, leaving the
isVisible (and I assume other) field null. Boom!

(cherry picked from commit commit ae942acf0db6db4404ea4d7b9c50d011552276a7)

Conflicts, which was git rm'ed:
	src/org/mythtv/service/channel/v28/ChannelHelperV28.java

Changes to be committed, from v1.30:
	modified:   src/org/mythtv/service/channel/v25/ChannelHelperV25.java
	modified:   src/org/mythtv/service/channel/v26/ChannelHelperV26.java
	modified:   src/org/mythtv/service/channel/v27/ChannelHelperV27.java