Per the resolution of POSIX bug 0000779 (note 0002050) add support for using 'i'

as a case insensitive flag.

PR:		standards/184641
Requested by:	David A. Wheeler <dwheeler@dwheeler.com>
MFC After:	1 week

usr.bin/sed/compile.c

@@ -582,6 +582,7 @@ compile_flags(char *p, struct s_subst *s)
 		case 'p':
 			s->p = 1;
 			break;
+		case 'i':
 		case 'I':
 			s->icase = 1;
 			break;

usr.bin/sed/sed.1

@@ -31,7 +31,7 @@
 .\"	@(#)sed.1	8.2 (Berkeley) 12/30/93
 .\" $FreeBSD$
 .\"
-.Dd May 24, 2009
+.Dd December 9, 2013
 .Dt SED 1
 .Os
 .Sh NAME
@@ -196,7 +196,7 @@ option was specified);
 a context address
 that consists of a regular expression preceded and followed by a
 delimiter. The closing delimiter can also optionally be followed by the
-.Dq I
+.Dq i
 character, to indicate that the regular expression is to be matched
 in a case-insensitive way.
 .El
@@ -507,7 +507,7 @@ Append the pattern space to
 if a replacement was made.
 If the replacement string is identical to that which it replaces, it
 is still considered to have been a replacement.
-.It I
+.It i or I
 Match the regular expression in a case-insensitive way.
 .El
 .Pp