move some information off mksh.1; harmonise

lksh.1

@@ -1,4 +1,4 @@
-.\" $MirOS: src/bin/mksh/lksh.1,v 1.12 2015/10/09 19:28:20 tg Exp $
+.\" $MirOS: src/bin/mksh/lksh.1,v 1.14 2015/10/09 20:33:49 tg Exp $
 .\"-
 .\" Copyright (c) 2008, 2009, 2010, 2012, 2013, 2015
 .\"	mirabilos <m@mirbsd.org>
@@ -173,6 +173,20 @@ It is recommended to port scripts to
 .Nm mksh
 instead of relying on legacy or idiotic POSIX-mandated behaviour,
 since the MirBSD Korn Shell scripting language is much more consistent.
+.Pp
+Note that it's strongly recommended to invoke
+.Nm
+with at least the
+.Fl o Ic posix
+option, if not both that
+.Em and Fl o Ic sh ,
+to fully enjoy better compatibility to the
+.Tn POSIX
+standard (which is probably why you use
+.Nm
+over
+.Nm mksh
+in the first place) or legacy scripts, respectively.
 .Sh LEGACY MODE
 .Nm
 currently has the following differences from
@@ -203,22 +217,30 @@ change between versions; see the accompanying manual page
 for the versions this document applies to.
 .It
 .Nm
-only offers the traditional ten file descriptors to scripts.
-.It
-.Nm
 uses
 .Tn POSIX
 arithmetics, which has quite a few implications:
-The data type for arithmetics is the host ISO C
+The data type for arithmetics is the host
+.Tn ISO
+C
 .Vt long
 data type.
-Signed integer wraparound is Undefined Behaviour.
+Signed integer wraparound is Undefined Behaviour; this means that...
+.Bd -literal -offset indent
+$ echo $((2147483647 + 1))
+.Ed
+.Pp
+\&... is permitted to, e.g. delete all files on your system
+(the figure differs for non-32-bit systems, the rule doesn't).
 The sign of the result of a modulo operation with at least one
 negative operand is unspecified.
 Shift operations on negative numbers are unspecified.
 Division of the largest negative number by \-1 is Undefined Behaviour.
 The compiler is permitted to delete all data and crash the system
-if Undefined Behaviour occurs.
+if Undefined Behaviour occurs (see above for an example).
+.It
+.Nm
+only offers the traditional ten file descriptors to scripts.
 .It
 .\"XXX TODO: move this to FPOSIX
 The rotation arithmetic operators are not available.
@@ -298,7 +320,18 @@ as
 .Pa /bin/sh ,
 compilation to enable
 .Ic set -o posix
-by default is highly recommended for better standards compliance.
+by default if called as
+.Nm sh
+is highly recommended for better standards compliance.
+For better compatibility with legacy scripts, such as many
+.Tn Debian
+maintainer scripts, Upstart and SYSV init scripts, and other
+unfixed scripts, using the compile-time options for enabling
+.Em both
+.Ic set -o posix -o sh
+when the shell is run as
+.Nm sh
+is recommended.
 .Pp
 .Nm
 tries to make a cross between a legacy bourne/posix compatibl-ish
@@ -308,7 +341,7 @@ is not exactly specified.
 .Pp
 The
 .Ic set
-built-in command does not have all options one would expect
+built-in command does not currently have all options one would expect
 from a full-blown
 .Nm mksh
 or