unix: silence STATIC_ASSERT compiler warnings

Fix the following two warnings:

  src/unix/core.c:74:1: warning: ISO C90 forbids array
  'static_assert_failed' whose size can't be evaluated [-Wvla]
  src/unix/core.c:76:1: warning: ISO C90 forbids array
  'static_assert_failed' whose size can't be evaluated [-Wvla]
linux: don't use fopen() in uv_resident_set_memory()

RSS is a reflection of the number of pages that a process has mapped.
glibc implements fopen() in terms of mmap() which means that trying
to read the number of mapped pages changes it. Switch to open().