Resolve conflicts with Guile primitives

The bind, filter and seek functions defined in lib.scm are renamed (to
bind-key, filter-library and seek-to, respectively) so that they do not
override functions from the standard environment.  The quit function
also overrides a Guile function, but is left as-is since the Guile quit
function should never be used in cmus.
String registers

Adds 256 character-addressed string registers, which may be specified in
format strings as '%$[char]'.  The commands 'set-register [reg] [value]'
and 'clear-register [reg]' set and clear a register, respectively.  The
command 'show-register [reg]' echos the value of the specified register.
Scheme functions (get-register [reg]) (set-register! [reg] [value]) and
(clear-register! [reg]) are also provided.
Hooks/callbacks

Scheme functions may be registered to execute in response to certain
events.  Currently the only available event (hook) is hook_file_changed,
which occurs whenever the the playing file changes.
current-track scheme function

(current-track) returns the currently playing track (player_info.ti).