Fix loading of multiline commands from vifmrc file

Those that contain bars ('|').
The issue was related to context switching of a command parsing
engine.  After initialization menu context is set.

Thanks to Sebastian Cyprych.
Fix trailing chars on loading of multiline cmds

From a vifmrc file when lines contain bars ('|').
The issue was related to processing of the \| sequence.
Extract skip_command_beginning() function

This reduces code duplication.
Merge branch 'fix-vifmrc-multiline-cmds'