Absorb platform detection hacks from jnr projects

[headius]

In jnr/jnr-enxio#39 we see the latest hack to detect platform, in this case detecting FreeBSD 12. A similar hack was done to jnr-posix, which contains many other such hacks to detect libc version on Linux and specific levels of *BSD. These platform-detection tricks are needed by nearly all consumers of jnr-ffi, so we should combine them and formalize the Platform API to be uniform for all jnr-ffi consumers.

[basshelal]

I can add the code from jnr/jnr-enxio#39 myself in some neat way it's just the code that checks if FreeBSD is version > 12 right?
Could add a new Platform instance to Platform.java called FreeBSD with a getVersion() method using the same hack logic, that way back in jnr/jnr-enxio#39 it would be as simple as:

if (Platform.getNativePlatform().getOS() == Platform.OS.FREEBSD) {
    FreeBSD freeBSD = (Platform.FreeBSD) Platform.getNativePlatform();
    int version = freeBSD.getVersion();
    if (version >= 12) {
        // do FreeBSD 12+ stuff
    } else {
       // do legacy stuff
    }
}

@headius Point me to the other hacks you're mentioning and I'll see if I can incorporate them in an elegant way as well. I won't be able to test any BSD code myself (never had any experience using a BSD) so I will have to assume the code I will bring in is correct.

[headius]

I poked around jnr-posix and found the following:

• The jnr-posix Platform class has a large number of "macros" for platform detection. Some of these are specific to POSIX or libc, but many (most?) are arguably library-agnostic and better suited to jnr-ffi: https://github.com/jnr/jnr-posix/blob/40396aa2bae7c6e18ae3d813f02d433c700e7e7d/src/main/java/jnr/posix/util/Platform.java
• The base platform detection does currently use jnr-ffi's Platform class: https://github.com/jnr/jnr-posix/blob/62c6a3bd6b7dd5507891ccab9bc2f8ff44d29a78/src/main/java/jnr/posix/POSIXFactory.java#L118-L146
• FreeBSD has separate detection for version, which is rather gross currently (shells out): https://github.com/jnr/jnr-posix/blob/752f608f5134e2c2eae125cda2e237cf4bbb4d36/src/main/java/jnr/posix/FreeBSDPOSIX.java#L52-L73
• Linux has detection logic for various things, but primarily the manifestation of the stat structure and functions (this is my number one hated detail about glibc): https://github.com/jnr/jnr-posix/blob/16bedc7bdd0a5e296f361244eed444de75f6c2ea/src/main/java/jnr/posix/LinuxPOSIX.java

Additionally, jnr-constants has some suspiciously similar macros and platform lookup logic here: https://github.com/jnr/jnr-constants/blob/1d006ac75cdbab78491f22796f3e84e5be918b71/src/main/java/jnr/constants/PlatformConstants.java

I looked over jnr-unixsocket and did not see anything that looked like platform detection.

[headius]

I should mention that some of the Linux platform detection logic is to differentiate glibc from e.g. musl, which I think is appropriate to leave in jnr-posix.

[basshelal]

Ok the version numbering hack has been added in #270