lavf/file: file_check: Handle file URLs that start with "file:"

Handle the URL analog to file_open, it may contain a "file:"
prefix. Skip it. Make access checks to file URLs starting
with "file:" work.

Fix part of ticket #3249.

Signed-off-by: Alexander Strasser <eclipse7@gmx.net>
doc/protocols/file: document general file protocol URL syntax

Also drop confusing ff* tools reference about exceptions to the
file:FILENAME syntax, which is not ff* tool specific.

With various edits by Alexander Strasser <eclipse7@gmx.net>.
doc/protocols/file: fix semantical reverse

Change "read" to "write" in introductory description.

Spotted-by: Alexander Strasser <eclipse7@gmx.net>
lavf/file: fix help message first character casing for trunc option
avformat/mp3dec: fix start time in light of initial skip samples

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>