Revise path handling to address issue #188

[erik-persson]

Purpose of this change:

• To make Oricutron runnable without cd'ing to its directory first
• To support relative paths with the -t option
• On macOS, to help command line users by not requiring a bundled app, allow "./oricutron" in the source directory much like under Linux.

Code changes:

• Factor out FILEPREFIX from ROMPREFIX and IMAGEPREFIX constants
• Apply FILEPREFIX for all filenames in oricutron.cfg which was done previously only for the keymap path
• Let function get_fileprefix() replace the FILEPREFIX constant, and use run-time concatenation instead of compile-time
• For Linux let get_fileprefix() return program binary's directory
• For macOS, let get_fileprefix() return parent of .app bundle if the program is bundled, otherwise do as in Linux.