Add 'geiser-connect-local' to connect over Unix-domain socket.
Highlight `define-once'

Guile's `define-once' allows defining a variable only once, but its
syntax is different from `define', so its highlight is different.
Disable error in `define-syntax-rule' font lock

This allows partially matched `define-syntax-rule' expressions and
avoids the termination of search-based fontification, which affects
other expressions inside the buffer, in the case of a missing
subexpression.
Fontify when too (thanks to Diogo)