Addressing comments.

src/core/support/env_posix.c

src/core/support/env_win32.c

src/core/support/file.h

src/core/support/file_posix.c

src/core/support/file_win32.c

test/core/end2end/fixtures/chttp2_simple_ssl_fullstack.c

test/core/support/file_test.c