glib: Rename G_STDIO_NO_WRAP_ON_UNIX → G_STDIO_WRAP_ON_UNIX

This clarifies the meaning a bit. Don’t change the logic associated with
it. Add a few comments to clarify things further.

Based on work done by Emmanuel Fleury.

Signed-off-by: Philip Withnall <withnall@endlessm.com>
Fixes: #77
9 jobs for <span class="ref-name">77-wrap-on-linux</span> in 15 minutes and 54 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #296407
cross-android_api21_arm64

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passed #296408
cross-android_api28_arm64

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passed #296409
cross-mingw64

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passed #296405
debian-stable-x86_64

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passed #296404
fedora-x86_64

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passed #296406
G_DISABLE_ASSERT

00:04:58

passed #296410
win32
msys2-mingw32

00:14:05

passed #296411
win32
vs2017-x64

00:09:08

 
  Coverage
passed #296412
coverage

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75.2%