Move libcap use to (Linux) platform-specific code

The libcap code is Linux-specific so move it all below
the linux/ platform subdirectory.  As this feature has
custom command-line long options I provide a mechanism
whereby each platform can add custom long options that
augment the main htop options.  We'll make use this of
this with the pcp/ platform in due course to implement
the --host and --archive options there.

Related to https://github.com/htop-dev/htop/pull/536

openbsd/Platform.h

@@ -70,4 +70,8 @@ static inline void Platform_getRelease(char** string) {
    *string = Generic_uname();
 }
 
+static bool Platform_getLongOption(ATTR_UNUSED int opt, ATTR_UNUSED int argc, ATTR_UNUSED char** argv) {
+   return false;
+}
+
 #endif