categories: Add os::linux-apis

Linux has a large API surface that isn't shared with other
members of the Unix/Unix-like family, so having a separate
category for it allows crate authors to better distinguish
when writing bindings for Linux-specific interfaces.

This is consistent with the availability of `os::macos-apis`,
as macOS is another member of the Unix/Unix-like family with
its own large and discrete API surface.

Author: woodruffw

src/boot/categories.toml

@@ -349,6 +349,12 @@ description = """
 Bindings to operating system-specific APIs.\
 """
 
+[os.categories.linux-apis]
+name = "Linux APIs"
+description = """
+Bindings to Linux-specific APIs.\
+"""
+
 [os.categories.macos-apis]
 name = "mac OS APIs"
 description = """