Allow user to set Rover-specific programs, e.g. ROVER_SHELL.

Show directory path while loading.

Refresh directory listing on SIGUSR1 signal.

Add config option to change default listing view flags.

Also escape single quotes inside shell argument.

Fix pathname splitting when running external programs.

Fix off-by-one error in readlink() call.

Check errors while trying to go to link target.

Add 't' to navigate to link target.

Add 'o' to open file with $ROVER_OPEN.

Refactor duplicate code into new function.

Remove cruft from usage text.

Make use of shell optional.

Launch external programs through shell. Passing $PAGER directly as argv[0] of a subprocess only works when $PAGER is simply the name of a binary file, such as "less". Users may want to hardcode command-line options, e.g. PAGER="less -N", so we need to do something like argv = {"sh", "-c", $PAGER}. The same applies to $SHELL and $EDITOR.

Fix deletion of single symlink-to-directory entry.