v0.2.1

Changelog:

• meta: improve efficiency of cross-platform interfaces on Solaris
• refactor: remove getpeereid()
• refactor(xucred): always define Xucred.pid
• fix(ffi): remove None check when loading libc (should fix on musl
  libc)
• fix(ucred): use correct socket level on NetBSD
• meta: fix packaging issues
• doc: improved documentation
• test: improved tests