realpath madness

Well, I've been bitten for the nth time by realpath(3)'s inability to 
understand context dependent symlinks (cdsl's).

(In this case it's sound-juicer trying to open the cdrom device).

So I propose to modify libc's realpath.

The dumb way would be to make libc depend on libcluster - but that seems 
insane, and hard.

So how to resolve cdsl's without using libcluster?

Well, it's easy, just make a cdsl:

/cluster/symlink -> /cluster/node{nodenum}/symlink/

and in /cluster/nodeXXX/symlink we have files containing the values of 
all the {xxx} things cdsl's can have,

i.e. /cluster/node6/symlink/nodenum contains "6".

realpath calls an "expand_cdsl" function after reading a symlink, which 
replaces {xxx} by the contents of /cluster/symlink/xxx (which is really 
/cluster/nodeXXX/symlink/xxx).

This even fixes a bug in my earlier ssi_realpath implementation - the 
"{nodenum}" we want is *not* the current nodenum, it's the "context" 
nodenum.

Testing underway.

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