Document POSIX tools with dgsh capabilities

[dspinellis]

This includes both to tools with modified source code and wrapped tools. The manual pages of wrapped tools with common I/O affordances (no input or no output) can be shared among them (e.g. dgsh-mute-command(1) and dgsh-deaf-command(1)). All other pages (e.g. dgsh-paste(1)) should list the dgsh differences and refer (via SEE ALSO) to the original page for more information.

Include these commands in the dgsh(1) SEE ALSO section.