PLT-294 Added slash command to check used version of CSH

[kivkiv12345]

With the recently added feature of the __FILE_DIR__ environment variable, it becomes apparent that CSH versions from before this commit will not work with scripts requiring the variable to be present.
This PR introduces a feature to help with such scenarios.

The new slash command: "require version csh" is useful in .csh scripts (init or not), where it can tell CSH to: exit, error or warn when a version constraint fails.

At the time of writing, the version constraint argument supports the typical comparisons: "==", "!=", ">=", "<=", ">" and "<".
But I have yet to add support for wildcard constraints, which would be useful for the patch version.

The name of the command is a bit clunky, as the intention is to also allow a command to check APM versions in the future.
Of course, if there are better suggestions for the name before merging, I would be happy to hear them.
The same can be said for the current error-level names: "fatal", "error" and "warn", where we may want to rename "fatal" to quit, so the all represent the resulting action.
I would like for the first letter of the error-level words to be different, as we currently allow single letters for short notation, of course this can also be debated.

[kivkiv12345]

Alright, this PR should be ready now (with wildcard support, unit tests and all). All we may want to do is add some more fail-cases for compare_version().

[kivkiv12345]

Just noticed that Valgrind compains an initialized value being passed to sscanf() here:

csh/src/require_version.c

Line 170 in ae87ba1

sscanf(constraint, "%d.%d.%d", &constraint_version.major, &constraint_version.minor, &constraint_version.patch);

, which is probably the constraint argument.
Either way it doesn't make much sense since both constraint and constraint_version are very much zero initialized:

csh/src/require_version.c

Lines 106 to 108 in ae87ba1

	char constraint[CONSTRAINT_MAXLEN] = {0};
	version_t constraint_version = {0, 0, 0};

[jeanbaptistelab]

LGTM

[troelsjessen]

Let us get it on master