relax banner delimiter and whitespace handling

I have IOS cfg files from copy run tftp operations. These are afflicted with

* unconditionally replaced banner delimiters with ^C (ETX, not Shift-6, C)
* embedded whitespace in banners (blank/whitespace only lines)

attempting to handle both "Shift-6, C"-style and control char-style delimiters and permit "blank" lines in banners by allowing CiscoConfParse constructor to tell IOSConfigList which chars shud be stripped from begin+end of lines