Fix build on Solaris 11.4 (re: d3cd4cf)

It was working on Solaris 11.3, but there were still problems
building on Solaris 11.4 with GCC (as on the evaluation VM
downloaded directly from Oracle):
1. ksh immediately segfaulted. Experimenting with the compiler
   flags Oracle uses revealed that we need to define _XPG6 for ksh
   not to segfault. Why is a mystery.
2. The default path logic used by 'command -p' and the 'getconf
   PATH' builtin command was still broken: the result did not
   include any of the /usr/xpg?/bin directories where the standard
   POSIX utilities actually live. Testing shows that the result of
   the C language probe 'confstr(_CS_PATH,name,length)' is broken
   on Solaris (it only yields the paths to the historic
   non-standard utilities, defeating the purpose) unless _XPG7 is
   defined; but the latter makes ksh segfault again. So another
   solution is needed.

src/cmd/INIT/package.sh, bin/package:
- Add another hack to add the -D_XPG6 flag to CCFLAGS if we're
  running SunOS aka Solaris. (I've tried to add a 'cc.sol11' script
  to src/cmd/INIT/ instead, but for some reason that I just don't
  have time to figure out, the INIT system ignores that on Solaris
  with gcc, so this is the only way I could come up with. Any
  patches for less hacky alternatives would be welcome.)

src/lib/libast/comp/conf.sh:
- Sanitise the code for finding the best 'getconf' utility.

src/lib/libast/comp/conf.tab: PATH:
- Since the C-languge getconf(_CS_PATH,...) is broken on Solaris
  11.4, replace the C language probe with a shell script probe that
  uses the external 'getconf' utility.
- To avoid ksh overriding the result of this probe with the result
  of its own getconf(_CS_PATH,...) call, which would make Solaris
  use the wrong value again, specify this as an AST configuration
  entry instead of a POSIX entry. This should be good enough for
  all systems; the OS 'getconf' utility should be reliable and the
  default path value is constant for each OS, so can be hardcoded.

src/cmd/ksh93/tests/builtins.sh:
- Add another 'sleep .1' to the 'sleep -s 31' test as it was still
  intermittently failing on Solaris and possibly other systems.

src/cmd/ksh93/tests/builtins.sh

@@ -876,6 +876,7 @@ cat >| "$sleepsig" << 'EOF'
 sleep -s 31 &
 sleep .1
 kill -CONT $!
+sleep .1
 if kill -0 $!; then
 	kill -TERM $! # Don't leave a lingering background process
 	exit 1