s3 make test: Add the ability to specify a custom smb.conf for make test

s3 make test: Add the ability to specify a custom smb.conf for make test

- Adds new -c <custom conf> option to selftest.sh that when specified
  adds a line to make test's server.conf: "include <custom conf>"
- Adds getopts processing to selftest.sh
- Changes selftest.sh shrdir arg to use -s <shrdir>
- Changes selftest.sh smbtorture4_path arg to use -t <smbtortur4 path>
- Adds configure option --with-selftest-custom-conf=<custom conf>
- Updates Makefile.in to take advantage of the new/changed parameters