PR11277: Use consistent octal in quoted strings

PR11277: Use consistent octal in quoted strings

Previously, our octal escapes used variable lengths, which can lead to
ambiguities.  Also, 8-bit characters would only output the least digit.

* runtime/string.c (_stp_text_str): Always output 3-digit octal escapes,
  and handle 8-bit chars more gracefully.
* testsuite/systemtap.string/text_str.stp: Include an 8-bit character.
* testsuite/systemtap.string/text_str.exp: Above + expect 3-digit octal.