upnp: fix browsing of certain servers

This is a partial revert of 785c6dfe. Some servers (notably PLEX, UMS,
and some TV sets) amend the exposed media type with additional subtypes
for further differentiation not relevant to VLC, so it is actually
correct to compare the beginning of the string only.

For instance "object.container" can turn into
"object.container.storageFolder" or "object.container.person.musicArtist"
so the plain strcmp will fail to recognize the container nature.

Fixes vlc-ios#1239

(cherry picked from commit 741f1f90)
11 jobs for 3.0.x in 16 minutes (queued for 10 seconds)
Status Name Job ID Coverage
  Build
passed android-arm #899192
docker amd64

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passed android-arm64 #899193
docker amd64

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passed android-x86 #899194
docker amd64

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passed android-x86_64 #899195
docker amd64

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passed debian #899188
docker amd64

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passed macos-arm64 #899191
amd64 catalina

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passed macos-x86_64 #899190
old-macmini

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passed snap #899189
docker amd64

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passed uwp64-libvlc-llvm #899187
docker amd64

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passed win32 #899185
docker amd64

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passed win64 #899186
docker amd64

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