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
030713fa26