Copy stable/10@r272459 to releng/10.1 as part of
the 10.1-RELEASE process.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Correct some typos in comments, no functional changes.

Check return value of dup(), it could be -1 when the system is running
out of file descriptors for instance.

Found with: Coverity Prevent(tm)
CID: 6084
MFC after: 1 month

Add support for uncompressing pack(1)'ed files. Pack(1) is a program found
in some commercial Unix systems, which utilizes Huffman minimum redundancy
code tree to compress files. This implementation supports the "new" pack
format only, just like GNU gzip did.

Thanks for oliver@'s archive set which I can test against, and Mingyan Guo
for providing helpful review of my code.

PR: bin/109567
MFC after: 1 month