Update src/core/s-trim.c

2013.02.02 by CCX (cc@x.isla.pr)

	>> trim "  x ^/"
	== "x x^/"

Fixed that trim bug, found in `trim_head_tail()`. Changed some var
names and the function declaration to make easier to understand
the algorithm. `head` and `tail` are now trimmed first because its
simpler that way.

First time I ever use github...