Fix 'command -p' by fixing initialisation of default PATH variable

'command -p' was broken for non-interactive shells as the variable
used to store the default system PATH, std_path, was not
initialised correctly. For instance:
	$ ksh -c 'command -p ls'
	ksh: ls: not found
This fix by Siteshwar Vashisht is backported from ksh2020.
Ref.:
https://github.com/att/ast/issues/426
https://github.com/att/ast/pull/448

src/cmd/ksh93/sh/path.c:
- Correctly initialise std_path (the default PATH) when ksh is
  started as a non-interactive shell.

src/cmd/ksh93/sh.1:
- Fix vague explanation of 'command -p'.

src/cmd/ksh93/tests/path.sh:
- Add regression test.

(cherry picked from commit a76439d60b70c18cf44d84c1962fcd8df84c947c)

NEWS

@@ -4,6 +4,13 @@ For full details, see the git log at:
 
 Any uppercase BUG_* names are modernish shell bug IDs.
 
+2020-05-19:
+
+- Fix 'command -p'. The -p option causes the operating system's standard
+  utilities path (as output by 'getconf PATH') to be searched instead of $PATH.
+  Before this fix, this was broken on non-interactive shells as the internal
+  variable holding the default PATH value was not correctly initialised.
+
 2020-05-16:
 
 - Fix 'test -t 1', '[ -t 1 ]', '[[ -t 1 ]]' in command substitutions.