make debugger expand relative paths to absolute

[trogper]

I have encountered issues with setting breakpoints in modules loaded using require, which uses absolute paths. Using relative paths brings problems in conjunction with the vscode extension, which might find the same file in multiple sourcesRoot directories. Absolute paths would solve this.

debugger class now has a public method set_working_dir, which accepts char*. If the method is not called, the old behaviour is preserved - relative paths are not expanded.

server class gets cwd in main and handles it to the debugger.

this PR is related to vscode-lrdb PR 13

[coveralls]

Coverage decreased (-5.7%) to 77.814% when pulling 41f7e5f999aaedafc7dd41eb7429becf51de1298 on kapecp:master into 68378984d689e7ca36b56a7c625bb5a6e3d318ba on satoren:master.