devel/shell-toolbox: Update to 20180719

Changelog:

• A skeleton directory (to pre-populate the temporary working directory with) may be specified by setting the SHELL_SKEL environment variable to the path of a directory. This environment variable is not used if the command line option -s or -d is used.
• /bin/sh is used if SHELL is empty or unset and no other shell is specified.

Reviewed by: mat
Approved by: mat (mentor)
Differential Revision: https://reviews.freebsd.org/D16714