docs: Link to plugin man pages from nbdkit(1).
server: Make the usage output more readable.
Change the open call to pass in the 'readonly' flag.

Not ABI stable quite yet, but soon ...
gitignore: Ignore gdb history file.
Add file plugin.
server: Add nbdkit_v{debug,error} va args variants.
server: Add gcc format attribute to detect errors in nbdkit_error/nbdkit_debug.

Also fix one actual bug.
server: If plugin data call doesn't set errno, return EIO.

Ensure that *error is set to something, else we do undesirable things
like reading garbage data or ignore writes without indicating to the
client that there was an error.
Add gzip plugin.