Rename my_chdir -> vifm_chdir
Prettify vifm_chdir() definition a bit
Add empty lines to utils/utils.h

For better readability.
Rename my_system -> vifm_system
Rename my_swprintf -> vifm_swprintf
Rename my_wcsdup -> vifm_wcsdup
Merge branch 'remove-my-prefixes'

Rename all my_foo() functions to vifm_foo().