unfill filter max doesn't match documentation

[Ben Collins]

In srec_examples, it shows "--unfill 0x00 512" as an example usage. However, srec_cat only allows the length portion (512 in this example) to be between 1 and 16.

a) The documentation and actual usage aren't matching

b) The hard coded maximum of 16 is entirely too short to be useful.

c) srecord has way too many arbitrary limits in it (both bugs I've filed are in some way related to an arbitrary, useless limit being placed). I suggest not limiting users and letting them shoot themselves in the foot if they want to.

Lastly, a feature request: Being able to limit --unfill to only apply to block aligned/sized segments (e.g. to align with flash sectors).