[main] Rewrite rootmkdir in C++

[silverweed]

This Pull request:

Replace the python version of rootmkdir with a native one.

Changes or fixes:

I changed one small thing from the original rootmkdir: calling:

rootmkdir foo.root:dir

when foo.root does not exist would create the file and the directory in the original, whereas in this version it will error out unless you pass the -p flag. The rationale is that the top-level file should count as a "parent directory", so we should not create it implicitly unless -p is specified. Calling rootmkdir foo.root works as before (except it will error out if the file already exists).

Checklist:

• tested changes locally
• updated the docs (if necessary)

[github-actions]

Test Results

    22 files      22 suites   3d 4h 57m 27s ⏱️
 3 828 tests  3 827 ✅ 1 💤 0 ❌
76 605 runs  76 596 ✅ 9 💤 0 ❌

Results for commit 481d29a.

♻️ This comment has been updated with latest results.

[vepadulano]

Cool thanks! Some minor comments from my side.

[pcanal]

What is the intent of the test? and/or how does it check for success?

[silverweed]

It verifies that calling rootmkdir file.root creates an empty ROOT file (rootls returns 1 if the file does not exist and 0 otherwise)