Update INSTALL.md to list 'which' as a required dependency.
Without it, scst init script fails with:
/etc/init.d/scst: line 149: which: command not found
as seen in systemd logs:
mp-scst-service scst[1198]: /etc/init.d/scst: line 149: which: command not found
mp-scst-service systemd[1]: mp-scst.service: Control process exited, code=exited, status=1/FAILURE
Stemming from the line:
https://github.com/SCST-project/scst/blob/v3.9/scstadmin/init.d/scst#L162
1. Correct a typo, changing `make release` to `make 2release`.
2. Add information about package-related targets to the help section.
3. Remove `dev_handler` directory during the uninstall process.
4. Eliminate the non-existent `tgt` target from the PHONY targets.
5. Introduce SCST_MOD_DIR and SCST_DH_MOD_DIR as helper variables.
