sysutils/squashfs-tools: update the port to version 4.5.1 (finally)

This is a long-awaited release which fixes known security issue [1]
and regression when working with extended attributes [2].

Install more complete documentation set while here and GC no longer
needed CFLAGS+= and USE_CSTD knobs.

Security: CVE-2021-41072 [1]
PR: 256790 [2], 261334