Fix multiple bugs in cam_strmatch

[asomers]

Summary

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

Test Plan

Added unit tests

[imp]

I suspect these same bugs are present in the file globbing code I stole the original implementation from.
Nice catches.

lib/libcam/tests/cam_test.c

91

There's no pattern like 'fo[a-z][a-z][123]' which has multiple ranges tested.

[asomers]

lib/libcam/tests/cam_test.c

91

Ok, I'll add one.