Fix multiple bugs in cam_strmatch

• Wrongly matches strings that are shorter than the pattern
• Fails to match negative character sets
• Fails to match character sets that aren't at the end of the pattern
• Fails to match character ranges

Reviewed by: imp
MFC after: 3 weeks
Sponsored by: Spectra Logic Corp
Differential Revision: https://reviews.freebsd.org/D13173