install_subst: helper script for installing with path substitution

A helper script to copy a source file substituting any
__ASTERISK_<foo>_DIR__ with the content of $AST<foo>DIR.

Review: https://reviewboard.asterisk.org/r/3202/


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@408059 f38db490-d61c-443f-a65b-d21fe96a405b