Avoid canonizing the same paths over and over.
Avoid canonizing the same paths over and over.

The number of path canonizations was quadratic in the number of potential
plugin directories. This patch makes it linear; on a standard Coq tree,
this brings the number of chdir and getcwd system calls from more than
1,000 down to about 200 at startup.

This also fixes a bug where the Cd vernacular command could cause plugins
to break since relative paths were used to locate them.

letouzey/coq-wip,coq/coq,ppedrot/coq,amintimany/coq,Ptival/coq,amintimany/coq,JasonGross/coq,ejgallego/coq,william-lawvere/coq,ppedrot/coq,SkySkimmer/coq,JasonGross/coq,amintimany/coq,Matafou/coq,letouzey/coq-wip,ppedrot/coq,coq/coq,letouzey/coq-wip,Lionel-Rieg/coq,letouzey/coq-wip,Lionel-Rieg/coq,JasonGross/coq,gares/coq,Zimmi48/coq,Zimmi48/coq,maximedenes/coq,silene/coq,JasonGross/coq,SkySkimmer/coq,coq/coq,maximedenes/coq,Ptival/coq,Zimmi48/coq,letouzey/coq-wip,Lionel-Rieg/coq,gares/coq,maximedenes/coq,ejgallego/coq,maximedenes/coq,silene/coq,gares/coq,maximedenes/coq,william-lawvere/coq,ppedrot/coq,coq/coq,silene/coq,Matafou/coq,Ptival/coq,ejgallego/coq,Zimmi48/coq,gares/coq,SkySkimmer/coq,gares/coq,ejgallego/coq,SkySkimmer/coq,william-lawvere/coq,ppedrot/coq,Matafou/coq