Fix various minor problems and update the documentation (#237)

These are minor fixes I've accumulated over time. The following
changes are somewhat notable:

- Added a missing entry for 'typeset -s' to the man page.
- Add strftime(3) to the 'see also' section. This and the date(1)
  addition are meant to add onto the documentation for 'printf %T'.
- Removed the man page the entry for ksh reading $PWD/.profile on
  login. That feature was removed in commit aa7713c2.
- Added date(1) to the 'see also' section of the man page.
- Note that the 'hash' command can be used instead of 'alias -t' to
  workaround one of the caveats listed in the man page.
- Use an 'out of memory' error message rather than 'out of space'
  when memory allocation fails.
- Replaced backticks with quotes in some places for consistency.
- Added missing documentation for the %P date format.
- Added missing documentation for the printf %Q and %p formats
  (backported from ksh2020: https://github.com/att/ast/pull/1032).
- The comments that show each builtin's options have been updated.

src/cmd/ksh93/mamexec

@@ -7,10 +7,10 @@
 #
 # @(#)mamexec (gsf@research.att.com) 07/17/94
 #
-# mamexec [ -dfins ] [ target ... ] < mam-file
+# mamexec [ -dfins ] [ target ... ] < mamfile
 #
 # NOTE: variables defined in this script may conflict with
-#	mam-file variables
+#	mamfile variables
 #
 
 _command_=mamexec
@@ -39,7 +39,7 @@ do	case $# in
 		;;
 	-*)	case $1 in
 		-*[abceghjklmopqrtuvwxyz_A-Z0-9]*) # [!...] not portable
-			echo "Usage: $_command_ [ -dfins ] [ target ... ] < mam-file" >&2; exit 2
+			echo "Usage: $_command_ [ -dfins ] [ target ... ] < mamfile" >&2; exit 2
 			;;
 		*)	case $1 in
 			*d*)	_debug_="eval echo $_command_: debug: >&2" ;;