[invoker] Clear POSIXLY_CORRECT after parsing the arguments

[turski]

POSIXLY_CORRECT environment variable is used for modifying getopt behaviour, but is not cleared before launching the application. This is causing some problems especially on terminals. See TJC thread over here: https://together.jolla.com/question/5561/make-fingertermbash-startup-execute-standard-profilebashrc-scripts/

[piral]

On jolla sailfish os it was possible to mount davfs over ssh but not over the terminal on jolla itself. As a workaround unset POSIXLY_CORRECT in a shell scipt will do the thing.

[amtep]

Invoker could use + at the start of the optstring instead of setting POSIXLY_CORRECT at all.

[special]

Fixed with @amtep's suggestion in #26

[special]

Fixed in mapplauncherd 4.1.15