Do not copy string to itself with copy_str()

This fixes crash on OS X 10.9 (I guess) which has runtime check for
misuse of memcpy() enabled.

Fixes #38 issue in ksteen/vifm.

Thanks to Daniel Dettlaff, a.k.a.  dmilith.
Skip setting same value in set_trash_dir()

Just return success code in this case.
Merge branch 'fix-new-osx-crash'

Fix crash on OS X 10.9 which has runtime check for misuse of
memcpy() enabled.

Thanks to Daniel Dettlaff, a.k.a.  dmilith.