MANPAGE: update exit codes
MANPAGE: update exit codes

The exit codes have been documented in CLI_ABI.md for a while,
but they should also be listed in the man page.

Also fix the rendering of "[-o COMMA-SEPARATED-OPTIONS]", where
the square brackets where interpreted as something. Escape all
square brackets to be safe.

rfjakob/gocryptfs,rfjakob/gocryptfs,rfjakob/gocryptfs,rfjakob/gocryptfs