Allow playback of formats that don't support seeking

ast_streamfile previously did unconditional seeking on files that broke
playback of formats that don't support that functionality.  This patch avoids
the seek that was causing the problem.  This regression was introduced in
r158062.

(closes issue ASTERISK-18994)
Patch-by: Timo Teras
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Merged revisions 349731 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 349732 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@349733 65c4cc65-6c06-0410-ace0-fbb531ad65f3