GNU Jobserver support

**Is your feature request related to a problem? Please describe.**

When running multiple `zstd` processes in parallel, it's difficult to know how many threads to give to each process. Giving each process as many threads as available results in oversubscription, given them only 1 may not be optimal use of resources.

**Describe the solution you'd like**

GNU Make has this concept of a jobserver, which ensures that `make -j ` doesn't run more than `n` jobs in parallel (recursively). It is widely supported (e.g. gcc's LTO spawns parallel jobs from gcc respecting the job server, clang, cargo, ...) and is trivial to implement: https://www.gnu.org/software/make/manual/html_node/POSIX-Jobserver.html.

This way, `make -j n` can be used to control the concurrency, and any `zstd` process could automatically take as many jobs as available to compress / decompress.

**Additional context**

This is particularly useful when Makefiles (or any other program conforming to the GNU Make jobserver) are used to create and compress multiple tarballs.