utils: Create ast_strsep function that ignores separators inside quotes

This function acts like strsep with three exceptions...
* The separator is a single character instead of a string.
* Separators inside quotes are treated literally instead of like separators.
* You can elect to have leading and trailing whitespace and quotes
stripped from the result and have '\' sequences unescaped.

Like strsep, ast_strsep maintains no internal state and you can call it
recursively using different separators on the same storage.

Also like strsep, for consistent results, consecutive separators are not
collapsed so you may get an empty string as a valid result.

Tested by: George Joseph
Review: https://reviewboard.asterisk.org/r/3989/
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Merged revisions 423476 from http://svn.asterisk.org/svn/asterisk/branches/12
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Merged revisions 423478 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@423480 65c4cc65-6c06-0410-ace0-fbb531ad65f3