trust: don't create file names longer then 255

[ZoltanFridrich]

trust extract creates file names based on CKA_LABEL attribute of trusted certificates. Some of these certificates contain labels that are longer then file name limit on linux which causes file creation to fail.
This MR makes make_unique_name to trim names that are equal or longer then 255.

[coveralls]

coverage: 69.543% (+0.001%) from 69.542%
when pulling cfbadae on ZoltanFridrich:zfridric_devel
into 89d3a55 on p11-glue:master.

[ueno]

Do we have any assurance that the truncated filename doesn't collide with others? Perhaps we could fall back to other naming scheme, e.g., hash, if the filename would be longer than 255?

[ZoltanFridrich]

Do we have any assurance that the truncated filename doesn't collide with others? Perhaps we could fall back to other naming scheme, e.g., hash, if the filename would be longer than 255?

Yes, we do. Because the name is truncated before any modifications and checks.

[ueno]

LGTM, looks like the uniqueness is ensured and also the generated file name ends with a NUL character. Thanks!