TS-3145: add additional textBuffer APIs

To make textBuffer more useful for formatting wawlls of text
  - fix textBuffer::enlargeBuffer so that it works with an initial size of 0
  - add textBuffer::format to append printf-formatted text
  - add textBuffer::chomp to remove any trailing newline
  - add textBuffer::release to remove the internal formatting buffer
TS-3145: minimally enhance ElevateAccess for different privileges

ElevateAccess only elevates the file access privilege. Add an
explicit notion of privilege so that we can use capabilities to
elevate other kinds of privilege (specifically, process tracing).
TS-3145: implement remote backtrace for traffic_server

Implement the ServerBacktrace API in traffic_manager using libunwind
and ptrace(2). This allow operators to log a back trace of all the
traffic_server threads.

Add the --backtrace option to traffic_line to display the backtrace.
TS-3145: CHANGES