Check return value from signal()

Error out if signal handler setup fails.

Fixes #246
options: reuse color name parser for obsolete color mappings
tree: add directory color type for use by draw_filename

This fixes coloring of directory names in the tree view by introducing a
new 'directory' color type used instead of 'tree-dir'. It also renames
existing 'filename' color to 'file' to make color names a bit more
consistent wrt. 'file-size'.
tree: use directory and file line types