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)

src/cmd/ksh93/sh.1

@@ -5806,7 +5806,8 @@ with the arguments given by
 The
 .B \-p
 option causes
-a default path to be searched
+the operating system's standard utilities path
+(as output by \f3getconf PATH\fP) to be searched
 rather than the one defined by the value of
 .SM
 .BR PATH .