Merge functional code from get_word() and get_word_sep() into a single

Merge functional code from get_word() and get_word_sep() into a single
function, since they both do approximately the same thing. At the same time,
remove redir_isspace() altogether since code elsewhere (including
get_word_sep()) uses strchr() for the same purpose. See summary in "[PATCH]
rtsp.c small cleanups" thread.

Originally committed as revision 18122 to svn://svn.ffmpeg.org/ffmpeg/trunk