Nheko is very chatty in its log output, generating log noise (which
complicates diagnostics) and needless disk writes (which affect power
consumption and SSD life).  This patch introduces command line options
and environment variables to control log levels and output type.

The old --debug command line option still works, at least for now.
It is overridden by the new command line options when they are used.

Partially addresses #665.

fixes #1030

This adds a man page which lists command line options, key bindings and
commands. A new CMake option `MAN` is added, enabled by default on
`UNIX` but not `APPLE`.

The man pages are generated with either `a2x` (from asciidoc) or
`asciidoctor`. If both are not found, a fatal error is signaled.