Remove SHOPT_SYSRC probe, enable by default

It's just not right that it depends on the presence of
/etc/ksh.kshrc or even /etc/bash.bashrec on the build machine
whether /etc/ksh.kshrc is going to be loaded at startup on the
users' machine.

src/cmd/ksh93/Mamfile:
- Remove SHOPT_SYSRC probe code.

src/cmd/ksh93/SHOPT.sh:
- Enable SHOPT_SYSRC by default.

README.md:
- Document that SHOPT_SYSRC should be disabled on systems with a
  default incompatible /etc/ksh.kshrc.

Resolves: #776

README.md

@@ -67,6 +67,12 @@ to package it as their default version of ksh93.
 To build ksh with a custom configuration of features, edit
 [`src/cmd/ksh93/SHOPT.sh`](https://github.com/ksh93/ksh/blob/dev/src/cmd/ksh93/SHOPT.sh).
 
+On systems such as NetBSD and OpenBSD, where `/bin/ksh` is not ksh93 and the
+preinstalled `/etc/ksh.kshrc` profile script is incompatible with ksh93, you'll
+want to disable `SHOPT_SYSRC` to avoid loading it on startup -- unless you can
+edit it to make it compatible with ksh93. This geneerally involves differences
+in the declaration and usage of local variables in functions.
+
 Then `cd` to the top directory and run:
 
 ```

src/cmd/ksh93/Mamfile

@@ -118,11 +118,6 @@ make install virtual
 		exec -			-zutalors)
 		exec -				writedef PRINTF_LEGACY 1 ;;
 		exec -			esac ;;
-		exec -		'SYSRC=')
-		exec -			# if one of these exists, make SHOPT_SYSRC load /etc/ksh.kshrc by default
-		exec -			if	test -f /etc/ksh.kshrc || test -f /etc/bash.bashrc
-		exec -			then	writedef SYSRC 1
-		exec -			fi ;;
 		exec -		# some other SHOPTs may be probed for using feature tests in features/options
 		exec -		*=?*)
 		exec -			writedef "$n" "$v" ;;

src/cmd/ksh93/README

@@ -146,9 +146,8 @@ The options have the following defaults and meanings:
 
     STATS        on  Add .sh.stats compound variable.
 
-    SYSRC            Source /etc/ksh.kshrc on initializing an interactive
-                     shell. This is on by default if /etc/ksh.kshrc or
-                     /etc/bash.bashrc exists at compile time.
+    SYSRC        on  Source /etc/ksh.kshrc on initializing an interactive
+                     shell.
 
     TEST_L           Add 'test -l' as an alias for 'test -L'. This is on by
                      default if the OS's external 'test' command supports it.

src/cmd/ksh93/SHOPT.sh

@@ -36,7 +36,7 @@ SHOPT REMOTE=				# enable --rc if running as a remote shell
 SHOPT SCRIPTONLY=0			# build ksh for running scripts only; compile out the interactive shell
 SHOPT SPAWN=				# use spawnveg for fork/exec
 SHOPT STATS=1				# add .sh.stats variable
-SHOPT SYSRC=				# attempt . /etc/ksh.kshrc if interactive
+SHOPT SYSRC=1				# attempt . /etc/ksh.kshrc if interactive
 SHOPT TEST_L=				# add 'test -l' as an alias for 'test -L'
 SHOPT TIMEOUT=				# number of seconds for shell timeout
 SHOPT VSH=1				# vi edit mode