ensure no duplicate XDG paths are prepended during startup

ensure no duplicate XDG paths are prepended during startup

the previous patch(es) had a number of issues which made them unsuitable for general use:

* only checking "/usr" and "/usr/local" paths, despite this only being accurate if e was installed into /usr or /usr/local

* only checking if the paths were at the beginning of the string, when it's possible that they could be anywhere

* failure to also check XDG_CONFIG_DIRS

* improper formatting: this is a bit of a nitpick, but there are no correct instances of 'strcmp(a, b) == 0' in the e codebase.